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.