Yes, that’s exactly what I was thinking. I’ve seen that happen before, and in that case it turned out to be a web API call that the handler was making that was never returning and didn’t have a timeout. When looking at a memory dump, I could see multiple stack traces that were sitting in that method.
If the message had finished processing, then the transport would have either sent the ack or it would have failed to send it and you’d see a log entry indicating that failure.
If you see this behavior again, collect a few memory dumps so that you can compare them to see if anything is changing over time. That should give you some insight into where the message processing was stuck.