I could swear that there was a thread in the not too long past that talked about a MQTT broker monitoring tool. But do you think I can find it...nooooo....drat's I thought I had it bookmarked.
Anyway If you can find it, it might shed some light on what the broker is doing.
In addition thei shread might help: Issues in persisting messages in MQTT Broker using NodeRed MQTT Nodes
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