The Zabbix proxy seems to be closing the UDP source port for SNMP checks before the timeout. In my zabbix_proxy.conf, I have Timeout=30, the max setting. However, you can see from the trace below that it sends an icmp port unreachable back to the monitored host for 6 of the SNMP queries:

02:48:44.436520 IP 172.19.2.134.51501 > 172.19.100.0.161: C=t0533k! GetRequest(32) .1.3.6.1.4.1.21695.1.2.2.4.1
02:48:45.435730 IP 172.19.2.134.51501 > 172.19.100.0.161: C=t0533k! GetRequest(32) .1.3.6.1.4.1.21695.1.2.2.4.1
02:48:45.458879 IP 172.19.2.134.36709 > 172.19.100.0.161: C=t0533k! GetRequest(33) .1.3.6.1.4.1.21695.1.2.2.4.74
02:48:46.170032 IP 172.19.2.134.57137 > 172.19.100.0.161: C=t0533k! GetRequest(33) .1.3.6.1.4.1.21695.1.2.2.4.75
02:48:46.436658 IP 172.19.2.134.51501 > 172.19.100.0.161: C=t0533k! GetRequest(32) .1.3.6.1.4.1.21695.1.2.2.4.1
02:48:46.459938 IP 172.19.2.134.36709 > 172.19.100.0.161: C=t0533k! GetRequest(33) .1.3.6.1.4.1.21695.1.2.2.4.74
02:48:47.170316 IP 172.19.2.134.57137 > 172.19.100.0.161: C=t0533k! GetRequest(33) .1.3.6.1.4.1.21695.1.2.2.4.75
02:48:47.218242 IP 172.19.2.134.55991 > 172.19.100.0.161: C=t0533k! GetRequest(33) .1.3.6.1.4.1.21695.1.2.2.4.76
02:48:47.439725 IP 172.19.2.134.51501 > 172.19.100.0.161: C=t0533k! GetRequest(32) .1.3.6.1.4.1.21695.1.2.2.4.1
02:48:47.461075 IP 172.19.2.134.36709 > 172.19.100.0.161: C=t0533k! GetRequest(33) .1.3.6.1.4.1.21695.1.2.2.4.74
02:48:48.171003 IP 172.19.2.134.57137 > 172.19.100.0.161: C=t0533k! GetRequest(33) .1.3.6.1.4.1.21695.1.2.2.4.75
02:48:48.219219 IP 172.19.2.134.55991 > 172.19.100.0.161: C=t0533k! GetRequest(33) .1.3.6.1.4.1.21695.1.2.2.4.76
02:48:48.440889 IP 172.19.2.134.51501 > 172.19.100.0.161: C=t0533k! GetRequest(32) .1.3.6.1.4.1.21695.1.2.2.4.1
02:48:48.462890 IP 172.19.2.134.36709 > 172.19.100.0.161: C=t0533k! GetRequest(33) .1.3.6.1.4.1.21695.1.2.2.4.74
02:48:48.479663 IP 172.19.2.134.51044 > 172.19.100.0.161: C=t0533k! GetRequest(33) .1.3.6.1.4.1.21695.1.2.2.4.77
02:48:49.171877 IP 172.19.2.134.57137 > 172.19.100.0.161: C=t0533k! GetRequest(33) .1.3.6.1.4.1.21695.1.2.2.4.75
02:48:49.223070 IP 172.19.2.134.55991 > 172.19.100.0.161: C=t0533k! GetRequest(33) .1.3.6.1.4.1.21695.1.2.2.4.76
02:48:49.441373 IP 172.19.2.134.51501 > 172.19.100.0.161: C=t0533k! GetRequest(32) .1.3.6.1.4.1.21695.1.2.2.4.1
02:48:49.463909 IP 172.19.2.134.36709 > 172.19.100.0.161: C=t0533k! GetRequest(33) .1.3.6.1.4.1.21695.1.2.2.4.74
02:48:49.481045 IP 172.19.2.134.51044 > 172.19.100.0.161: C=t0533k! GetRequest(33) .1.3.6.1.4.1.21695.1.2.2.4.77
02:48:49.513851 IP 172.19.2.134.36436 > 172.19.100.0.161: C=t0533k! GetRequest(33) .1.3.6.1.4.1.21695.1.2.2.4.78
02:48:50.173564 IP 172.19.2.134.57137 > 172.19.100.0.161: C=t0533k! GetRequest(33) .1.3.6.1.4.1.21695.1.2.2.4.75
02:48:50.223444 IP 172.19.2.134.55991 > 172.19.100.0.161: C=t0533k! GetRequest(33) .1.3.6.1.4.1.21695.1.2.2.4.76
02:48:50.464869 IP 172.19.2.134.36709 > 172.19.100.0.161: C=t0533k! GetRequest(33) .1.3.6.1.4.1.21695.1.2.2.4.74
02:48:50.482319 IP 172.19.2.134.51044 > 172.19.100.0.161: C=t0533k! GetRequest(33) .1.3.6.1.4.1.21695.1.2.2.4.77
02:48:50.515081 IP 172.19.2.134.36436 > 172.19.100.0.161: C=t0533k! GetRequest(33) .1.3.6.1.4.1.21695.1.2.2.4.78
02:48:51.174131 IP 172.19.2.134.57137 > 172.19.100.0.161: C=t0533k! GetRequest(33) .1.3.6.1.4.1.21695.1.2.2.4.75
02:48:51.224477 IP 172.19.2.134.55991 > 172.19.100.0.161: C=t0533k! GetRequest(33) .1.3.6.1.4.1.21695.1.2.2.4.76
02:48:51.483703 IP 172.19.2.134.51044 > 172.19.100.0.161: C=t0533k! GetRequest(33) .1.3.6.1.4.1.21695.1.2.2.4.77
02:48:51.517015 IP 172.19.2.134.36436 > 172.19.100.0.161: C=t0533k! GetRequest(33) .1.3.6.1.4.1.21695.1.2.2.4.78
02:48:52.226624 IP 172.19.2.134.55991 > 172.19.100.0.161: C=t0533k! GetRequest(33) .1.3.6.1.4.1.21695.1.2.2.4.76
02:48:52.489389 IP 172.19.2.134.51044 > 172.19.100.0.161: C=t0533k! GetRequest(33) .1.3.6.1.4.1.21695.1.2.2.4.77
02:48:52.521921 IP 172.19.2.134.36436 > 172.19.100.0.161: C=t0533k! GetRequest(33) .1.3.6.1.4.1.21695.1.2.2.4.78
02:48:53.004966 IP 172.19.100.0.161 > 172.19.2.134.51501: C=t0533k! GetResponse(33) .1.3.6.1.4.1.21695.1.2.2.4.1=9
02:48:53.005064 IP 172.19.2.134 > 172.19.100.0: icmp 85: 172.19.2.134 udp port 51501 unreachable
02:48:53.024505 IP 172.19.100.0.161 > 172.19.2.134.51501: C=t0533k! GetResponse(33) .1.3.6.1.4.1.21695.1.2.2.4.1=9
02:48:53.024572 IP 172.19.2.134 > 172.19.100.0: icmp 85: 172.19.2.134 udp port 51501 unreachable
02:48:53.027669 IP 172.19.100.0.161 > 172.19.2.134.36709: C=t0533k! GetResponse(34) .1.3.6.1.4.1.21695.1.2.2.4.74=16
02:48:53.027717 IP 172.19.2.134 > 172.19.100.0: icmp 86: 172.19.2.134 udp port 36709 unreachable
02:48:53.030983 IP 172.19.100.0.161 > 172.19.2.134.57137: C=t0533k! GetResponse(34) .1.3.6.1.4.1.21695.1.2.2.4.75=116
02:48:53.031030 IP 172.19.2.134 > 172.19.100.0: icmp 86: 172.19.2.134 udp port 57137 unreachable
02:48:53.034499 IP 172.19.100.0.161 > 172.19.2.134.51501: C=t0533k! GetResponse(33) .1.3.6.1.4.1.21695.1.2.2.4.1=9
02:48:53.034560 IP 172.19.2.134 > 172.19.100.0: icmp 85: 172.19.2.134 udp port 51501 unreachable
02:48:53.037619 IP 172.19.100.0.161 > 172.19.2.134.36709: C=t0533k! GetResponse(34) .1.3.6.1.4.1.21695.1.2.2.4.74=16
02:48:53.037677 IP 172.19.2.134 > 172.19.100.0: icmp 86: 172.19.2.134 udp port 36709 unreachable
02:48:53.040789 IP 172.19.100.0.161 > 172.19.2.134.57137: C=t0533k! GetResponse(34) .1.3.6.1.4.1.21695.1.2.2.4.75=116
02:48:53.044104 IP 172.19.100.0.161 > 172.19.2.134.55991: C=t0533k! GetResponse(34) .1.3.6.1.4.1.21695.1.2.2.4.76=42
02:48:53.046837 IP 172.19.100.0.161 > 172.19.2.134.51501: C=t0533k! GetResponse(33) .1.3.6.1.4.1.21695.1.2.2.4.1=9
02:48:53.050833 IP 172.19.100.0.161 > 172.19.2.134.36709: C=t0533k! GetResponse(34) .1.3.6.1.4.1.21695.1.2.2.4.74=16
02:48:53.054693 IP 172.19.100.0.161 > 172.19.2.134.57137: C=t0533k! GetResponse(34) .1.3.6.1.4.1.21695.1.2.2.4.75=116
02:48:53.058442 IP 172.19.100.0.161 > 172.19.2.134.55991: C=t0533k! GetResponse(34) .1.3.6.1.4.1.21695.1.2.2.4.76=42
02:48:53.063419 IP 172.19.100.0.161 > 172.19.2.134.51501: C=t0533k! GetResponse(33) .1.3.6.1.4.1.21695.1.2.2.4.1=9
02:48:53.067511 IP 172.19.100.0.161 > 172.19.2.134.36709: C=t0533k! GetResponse(34) .1.3.6.1.4.1.21695.1.2.2.4.74=16
02:48:53.071559 IP 172.19.100.0.161 > 172.19.2.134.51044: C=t0533k! GetResponse(34) .1.3.6.1.4.1.21695.1.2.2.4.77=14
02:48:53.074582 IP 172.19.100.0.161 > 172.19.2.134.57137: C=t0533k! GetResponse(34) .1.3.6.1.4.1.21695.1.2.2.4.75=116
02:48:53.077709 IP 172.19.100.0.161 > 172.19.2.134.55991: C=t0533k! GetResponse(34) .1.3.6.1.4.1.21695.1.2.2.4.76=42
02:48:53.080822 IP 172.19.100.0.161 > 172.19.2.134.51501: C=t0533k! GetResponse(33) .1.3.6.1.4.1.21695.1.2.2.4.1=9
02:48:53.084384 IP 172.19.100.0.161 > 172.19.2.134.36709: C=t0533k! GetResponse(34) .1.3.6.1.4.1.21695.1.2.2.4.74=16
02:48:53.089893 IP 172.19.100.0.161 > 172.19.2.134.51044: C=t0533k! GetResponse(34) .1.3.6.1.4.1.21695.1.2.2.4.77=14
02:48:53.106620 IP 172.19.100.0.161 > 172.19.2.134.36436: C=t0533k! GetResponse(34) .1.3.6.1.4.1.21695.1.2.2.4.78=17
02:48:53.112380 IP 172.19.100.0.161 > 172.19.2.134.57137: C=t0533k! GetResponse(34) .1.3.6.1.4.1.21695.1.2.2.4.75=116
02:48:53.118134 IP 172.19.100.0.161 > 172.19.2.134.55991: C=t0533k! GetResponse(34) .1.3.6.1.4.1.21695.1.2.2.4.76=42
02:48:53.122716 IP 172.19.100.0.161 > 172.19.2.134.36709: C=t0533k! GetResponse(34) .1.3.6.1.4.1.21695.1.2.2.4.74=16
02:48:53.128423 IP 172.19.100.0.161 > 172.19.2.134.51044: C=t0533k! GetResponse(34) .1.3.6.1.4.1.21695.1.2.2.4.77=14
02:48:53.135297 IP 172.19.100.0.161 > 172.19.2.134.36436: C=t0533k! GetResponse(34) .1.3.6.1.4.1.21695.1.2.2.4.78=17
02:48:53.139880 IP 172.19.100.0.161 > 172.19.2.134.57137: C=t0533k! GetResponse(34) .1.3.6.1.4.1.21695.1.2.2.4.75=116
02:48:53.143976 IP 172.19.100.0.161 > 172.19.2.134.55991: C=t0533k! GetResponse(34) .1.3.6.1.4.1.21695.1.2.2.4.76=42
02:48:53.148365 IP 172.19.100.0.161 > 172.19.2.134.51044: C=t0533k! GetResponse(34) .1.3.6.1.4.1.21695.1.2.2.4.77=14
02:48:53.154319 IP 172.19.100.0.161 > 172.19.2.134.36436: C=t0533k! GetResponse(34) .1.3.6.1.4.1.21695.1.2.2.4.78=17
02:48:53.159240 IP 172.19.100.0.161 > 172.19.2.134.55991: C=t0533k! GetResponse(34) .1.3.6.1.4.1.21695.1.2.2.4.76=42
02:48:53.162700 IP 172.19.100.0.161 > 172.19.2.134.51044: C=t0533k! GetResponse(34) .1.3.6.1.4.1.21695.1.2.2.4.77=14
02:48:53.166749 IP 172.19.100.0.161 > 172.19.2.134.36436: C=t0533k! GetResponse(34) .1.3.6.1.4.1.21695.1.2.2.4.78=17

I tried increasing the StartPollers to 10 from the default 5, but same behavior.