there was a reported case Filebeat Autodiscover Hints Breaking Template
when allowing hints started emitting of logs from all the containers even system ones. So I guess if these are emitted and log_topic is generated only for the ones matching condition then Kafka topic fallback to empty for others and therefore the error message. This is my guess.
I guess you cannot identify the source of the message. maybe you can play a little with hints.enabled to see if it helps