Considering you are not using the Flatpak anymore it is, indeed, strange. The only reasons I can think of are: your network manager is using the wrong network interface to route your traffic ( if you go on an ip checking site like for example ipinfo do you see yours or the VPN’s IP?) or that you have WebRTC enabled and the broadcaster is getting your real ip through that.
For the first case it can get pretty complicated, but it is probably an error during the installation of the VPN app or you set up multiple network managers and it gets confused on which one to configure. You should also enable the Advanced Kill Switch in the configuration.
For the second case you could try adding something like the Disable WebRTC add-on for firefox and check if it works. Remember to enable it for Private Windows too.
The last thing I can think of is that you allowed the broadcaster to get your real geolocation (in firefox it should be a small icon on the left of the address bar), or you are leaking some kind of information somewhere: there are a bunch of site that check for ip leak, but I don’t know if that goes too deep for you.
If you want to check anyway the first two results from DDG are browserleaks and ipleak. Mullvad offered one too but it is currently down.
EDIT: If you enable the Advanced Kill Switch, and the app is working correctly, internet will not work while you are not connected to a VPN server or until you disable the switch again, so pay attention to that.