You can start Bitcoin Core with the -debug=net option and find and post any lines beginning with pcp: or natpmp:. This will tell us what that module is doing, and should inform us of the issue.
It would still be useful to have this information to be sure it’s not an issue at the OS level or an implementation bug. Also add the lines with
portmap:.
(feel free to DM it as it may include privacy sensitive information)
my BTC Core application was unable to accept incming connections any more. But that is the most important feature for me, as i consider supporting the great BTC network as an exclusively important goal.
Another option, that always works, would be to accept incoming connections on Tor through an onion service, this works even if no port forwarding can be set up at the TCP level.