When items with key modbus.get on a host start experiencing timeouts, the memory usage by Zabbix Agent 2 process begins to grow. The more timeouts there are, the greater memory usage.
Memory utilization grows until the agent is restarted or until usage reaches 100%. After restarting the agent, memory usage returns to normal levels, but the issue reappears once timeouts start happening again.
Zabbix agent logs with levels 4 and 5 only contains entries about creaion of requests:
executing direct exporter task for key 'modbus.get
adding new request for key: 'modbus.get