Issue already posted in the zabbix forum:

https://www.zabbix.com/forum/zabbix-help/489802-agent2-mqtt-plugin-issue-time-out-while-subscribing

Steps to reproduce:

1. template with a master item (use postprocessing to format the data) that subscribe to an mqtt topic with 40+ items
2. child items that depend on that master item (use postprocessing to extract the data)
3. have two or more hosts which apply this template
4. MQTT broker, may be on the same or a different system

See template file...

Result:
See screenshot...
See log file...

Remarks:

The issue exists also in lower versions.

There the amount of items per host, before having this issue, is much higher (approx. 1000).