Plex has a quiet plan B called Relay. When your server can't be reached from the internet directly, Plex bounces the stream through its own servers so playback still works. Nobody gets an error. Everybody gets 720p.
Relay only appears when direct remote access is broken, so the fix is to repair the direct path. Start with turning on Remote Access. If that ends in a red X, continue to port forwarding.
Eyeball spots this automatically, by the way. A watched server that slips into Relay gets flagged as limited quality on your dashboard, so you find out before movie night does.
Eyeball checks your Plex from the internet every minute and alerts you when it breaks. It's free, and setup is one sign-in.