Let's say the following is found in your main RabbitMQ log.
2021-07-07 00:03:35.686 [warning] <0.849.0> Message dropped. Dead-letter queues cycle detected: [<<"foo.queue">>,<<"bar.queue">>,<<"foo.queue">>] This cycle will NOT be reported again.
This can occur when the original queue and the dead letter queue are both configured to move a message to another exchange when Time to Live (TTL) period has expired. Let's say foo.queue and bar.queue are both configured with a TTL of 60000 miliseconds (that's 1 minute).
According to https://www.rabbitmq.com/dlx.html, the message will be dropped when the message is moved to a dead letter exchange without a dead letter routing key as long as the message was not rejected.