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.