I found the same exact problem with ONE of my timelines but not the others. When navigating to the other timelines the language would switch back to the proper one selected in the language setup menu.
I have 6 subtitle tracks in 6 different languages in my project. With one timeline, only 3 languages could appear, and not necessarily the one selected. So for that timeline if I selected subtitles off that worked. Selecting language 1 or 2 on this timeline would show language 1. Selecting language 3 or 4 would show language 2. Selecting language 5 or 6 would show language 3.
I found that I had an extra, 7th subtitle track at the bottom that was blank. I was testing something and forgot to delete it. When I deleted that and re-saved the project the subtitles tracked properly. Not sure if that’s your problem but it sounds similar.
Tim