Issues with subtitles when using browsers

I rip my own blu-rays. I convert them all to m4v (mp4) and add information to the files using Subler. I use OCR to convert my subtitles from the blu-ray to .srt. However, after I’ve added the movies to my Plex server, I get something weird. When I watch the movies on my AppleTV or my iOS devices using the Plex app, everything works as expected but as soon as I try to watch in a browser, no matter what operating system, I get these giant subtitles. When I turn off the subtitles, then I get the subtitles the way I’d want them.
I’ve been trying to figure out what’s going on here for a while but so far, I have no idea.

If you mean with ‘a browser’ = Safari, look at this:

Do also take a look into
Settings - Web - Player -
Subtitle Size
and
Burn subtitles
try different values