Part of the issue is the fact that PGS and VOBSUB subtitles are actually graphics. And sometimes they don’t have just the dimensions of the letters themselves, but are the same size as the video picture (with all pixels except the actual text being transparent).
If you rip such a disc and then crop the video picture to remove black borders (or overall scale it down), the graphics of the subtitles and the video picture don’t fit each other anymore.
How this situation is handled during playback can differ across platforms, and is sometimes dependent on various additional factors, e.g. how the specific file is set up etc.
The safest way is to change “picture” subtitles into actual “text” subtitles. i.e. turn them into the SRT format. Those don’t care about the pixel dimensions of the video picture, but are rendered according to general preferences of each player. Which then allows you to scale them differently for large vs. small screen sizes, or change their color, or add a border or a background.
