There have no changes between 1.15.4.993-bb4a2cb6c and 1.15.4.994-107756f7e relating to remote access.
With regards to setting up port when manually specifying the port in the server remote access settings, it should not be a Port Triggering rule but a Port Forwarding rule.
Could you get me a logs from 1.15.4.993 with debug enabled and a restart and displaying the remote access settings page and then capturing the logs
and try 1.15.4.994 again and if the problem is there to capture the logs and attach both sets
Thanks
See
https://support.plex.tv/articles/201643703-reporting-issues-with-plex-media-server/
https://support.plex.tv/articles/200250417-plex-media-server-log-files/