When the dead letter queue reaches the size defined by dead_letter_queue.max_bytes, additional dead letters directed to this queue will cause logstash to emit:

cannot write event to DLQ(path: /var/lib/logstash/dead_letter_queue/main): reached maxQueueSize of <N>

It is indicated to upstream that the dead letter queue feature still has some improvements to be made: https://github.com/elastic/logstash/issues/8795

The core of the issue is that currentQueueSize never checks the size of the queue on disk (except at startup), but is perpetually increased by the size of each message that gets placed on the queue: https://github.com/elastic/logstash/blob/v7.10.0/logstash-core/src/main/java/org/logstash/common/io/DeadLetterQueueWriter.java#L179