This issue was uncovered while investigating
The conflation behaviour of last value queues appears correct from the perspective of consumers. However, the message count suggests that conflation is not occurring.
I checked what happens when ConflationQueueList.add(..) is replacing an existing ConflationQueueEntry. ConflationQueueList calls acquire() on the entry and then tries to discard it. The discard does nothing because it tries again to acquire the message. Therefore, the message stays in the list, causing a memory leak.
Because the message has been acquired, it is not delivered to consumers - that is why everything appears to work correctly from their point of view.