Check if there are several subtitle streams available for the video. It might be that the client is playing a different than the selected one. (Which might be an image-based one, so you can’t format it.)
Confirming there are often multiple subtitle streams muxed into my files.
You just reminded me it could have been the Android bug where any muxed-in ASS subs trigger Plex to ignore external subs. Unfortunately dashboard still says external SRT is playing.
There is no reason to trust the dashboard in this case. If even the app doesn’t know which subtitle it actually plays, how is the dashboard supposed to know better?