Ugh yeah, I ran into that previously. In fact a post on that is still on the jellyfin subreddit from some time ago. It was real annoying to figure out.
That will get fixed eventually, it took plex years to get ASS support working properly, its a complex format to render client side, particularly when its doing the fancy moving overlay stuff to pull of tricks like replacing text on a poster or sign and such. Its one of the few things still holding me from totally switching over to Jellyfin.
Turns out it's the subtitles. As I watch a lot of anime it's either good subtitles and transcode, or bad subtitles and direct. As the extra styling is not covered by the application
It's true I could change it to external player but then you lose half the features...
For instances like this, you could either use the Jellyfin plugin that extracts embedded subtitles, such that they are stored in an .art file and don't need transcoding, or set up Bazarr for the same (amomg handing all your subtitle needs).
I found that I preferred Bazarr, especially with notifications through Gotify, as I would directly see what took place when a subtitle was pulled, where as the Jellyfin plugin felt a bit like a black box.