Counted UPnP forwards from my router, found 79 entries. I have 80 nodes running. One of them had 0 connections, and it’s port number matches with the missing UPnP forward.
Router log has this entry:
Mar 20 19:45:56 miniupnpd[2020]: removed 1 unused rules
Node has been running for 36 hours. Maybe the UPnP forward should be renewed at some points?
But for now, I’ll change back to manually forwarded ports.