Note: I'm reporting this as a documentation issue, but it may be a software bug instead. Please see the last paragraph.

https://www.zabbix.com/documentation/current/en/manual/config/items/itemtypes/snmptrap, in the "Overview" section, states that the source address of an SNMP trap is matched against either the IP or DNS address of an SNMP interface, depending on which addressing mode it is set to.

In the next section, "Configuring SNMP traps", however, this sentence appears:

The address from each received trap is compared to the IP and DNS addresses of all SNMP interfaces to find the corresponding hosts.

The following section, "Setting up SNMP trap monitoring", also reads as if both IP and DNS names are looked up:

[...] where [address] will be compared to IP and DNS addresses of SNMP interfaces on Zabbix.

The current behavior matches what is in the Overview section; the other two descriptions contradict it.

However, I think this behavior is not well thought out and should actually be replaced with what is in the other sections, i.e. both IP and DNS names should be matched against. This was the behavior prior to ZBX-5797. It was changed to the current logic apparently in response to a report that traps were processed twice if the IP address was also entered into the DNS field. A better fix back then would have been to avoid adding a second identical entry into the lookup list of SNMP interface addresses (or even just documenting the behavior). As things are now, there is a dependency between the Zabbix and, say, SNMPTT configurations because they must match in which address they put into the formatted trap. Moreover, the Zabbix manual instructs the user to always use $aA (IP address) in snmptt.conf, but does not mention that the SNMP interface must be configured to use the IP address as well.