I have seen this happen in the other direction as well… When you watch a 1080p or 4k video with .sub .idx (DVD) subtitles the subs looks blurry. While .srt, or .sup look nice and crisp.
I know this happens because .sub basically contains pictures of the text in DVD resolution that are placed under the main video you are playing. Meanwhile .srt is just a text directory with time stamps
But maybe there is a way to make it look nicer ?