Steps to reproduce:

1. Create a host with two SNMP interfaces - SNMPv2c and SNMPv3. 
2. Link an SNMP item through a template
3. Add a host interface with SNMPv1.
4. Change SNMPv1 to a default interface.
5. Reload config cache on both server and then proxy.
6. 'Execute now' on an item with type SNMP.
7. Still see timeouts in zabbix_proxy.log:

6177:20200915:151416.976 getting SNMP values failed: Timeout while connecting to "172.27.73.54:161".
6181:20200915:151416.991 getting SNMP values failed: Timeout while connecting to "172.27.73.54:161".
6178:20200915:151417.998 getting SNMP values failed: Timeout while connecting to "172.27.73.54:161".
6175:20200915:151422.470 getting SNMP values failed: Timeout while connecting to "172.27.73.54:161".
6176:20200915:151422.559 getting SNMP values failed: Timeout while connecting to "172.27.73.54:161".
6179:20200915:151422.568 getting SNMP values failed: Timeout while connecting to "172.27.73.54:161".
6182:20200915:151422.590 getting SNMP values failed: Timeout while connecting to "172.27.73.54:161".

SNMPwalk from CLI is working:

[root@zabbix-proxy-btc-1 maksimse]# snmpwalk -v1 -ccommunity 172.27.73.54 sysDescr
SNMPv2-MIB::sysDescr.0 = STRING: Linux 2.6.32.68 #1 Tue Jun 23 16:48:07 EEST 2020 mips

And, by the way, your documentation is wrong:

https://www.zabbix.com/documentation/current/ru/manual/config/items/itemtypes/snmp

Choose version 5.0 and see 1st screenshot. There is SNMPv3 item, but there is not such item type in 5.0 anymore.