Entries since first unacked message

[filthz]

Hi,

from time to time we see this:
There seems to be a message that was not acked. That blocks the ledger from being deleted as no TTL is set in order no to lose any messages in case the consuming service is not up.

Restarting the consuming service usually does fix the situation

What can cause this situation and how can we solve it?
Why is there a non-acked message while the backlog on the queue is 0?
Why is the message not discarded?