I have host A and B, and Zabbix server Z, everything version 3.4.11. Host B have item trapper_item_key with type Zabbix Trapper, Allowed hosts might be set to Z or stay unset, it doesn't affect anything at all. Scenarios:

1. Not encrypted connection between A and Z, B and Z.

     A$ zabbix_sender -k trapper_item_key -o 0 -s B -z Z
     A$ zabbix_sender -k trapper_item_key -o 0 -s B -c /etc/zabbix/zabbix_agentd.conf
     ### Expected:
     # info from server: "processed: 1; failed: 0; total: 1; seconds spent: 0.000018"
     ### Actual:
     # info from server: "processed: 1; failed: 0; total: 1; seconds spent: 0.000018"
     

1. Encrypted connection (PSK) between A and Z, not encrypted connection between B and Z.

     A$ zabbix_sender -k trapper_item_key -o 0 -s B -z Z
     ### Encryption is not used for this connection due to only -z option passed.
     ### Expected:
     # info from server: "processed: 1; failed: 0; total: 1; seconds spent: 0.000018"
     ### Actual:
     # info from server: "processed: 1; failed: 0; total: 1; seconds spent: 0.000018"
     A$ zabbix_sender -k trapper_item_key -o 0 -s B -c /etc/zabbix/zabbix_agentd.conf
     ### Encryption is is used because it's settings in configuration, PSK settings for host A
     ### Expected:
     # info from server: "processed: 1; failed: 0; total: 1; seconds spent: 0.000018"
     ### Actual:
     # info from server: "processed: 0; failed: 1; total: 1; seconds spent: 0.000018"
     

1. Encrypted connection (PSK) between A and Z, B and Z, keypairs for A and B are different

     A$ zabbix_sender -k trapper_item_key -o 0 -s B -z Z
     ### Encryption is not used for this connection due to only -z option passed.
     ### Expected:
     # info from server: "processed: 1; failed: 0; total: 1; seconds spent: 0.000018"
     ### Actual:
     # info from server: "processed: 0; failed: 1; total: 1; seconds spent: 0.000018"
     A$ zabbix_sender -k trapper_item_key -o 0 -s B -c /etc/zabbix/zabbix_agentd.conf
     ### Encryption is is used because it's settings in configuration, PSK settings for host A
     ### Expected:
     # info from server: "processed: 1; failed: 0; total: 1; seconds spent: 0.000018"
     ### Actual:
     # info from server: "processed: 0; failed: 1; total: 1; seconds spent: 0.000018"
     

All scenarios with processed: 0 items are actual problems. It seems to me what Zabbix Server incorrectly checking PSK for trapper items on host with enabled PSK encryption when data coming from some other host without encryption or with other PSK key.