External subtitle slowly go out-of-sync on 10.9

[Eisaichen]

Describe The Bug
External subtitles will slowly go out of sync and get worse while playing continues.
It's synced when the playing just started. subtitle files are good, it is synced when playing with other players and also synced on 10.8.13.
fallback font is used. won't have this issue if transcoding.
It started about two months ago with 10.9 unstable build.
Subtitle type: ASS
Video for test: https://1drv.ms/u/s!AmrsUQ8bL7octplXoHqfBMdYMJSurg?e=ZQcTEj

Steps To Reproduce

1. Play a video that is longer than an hour.
2. Enable External subtitle
3. Play it for 10 minutes or longer
4. Check the subtitle to see if it's still in sync

Expected Behavior
Subtitles are in sync for the entire time.

Logs
No errors

System (please complete the following information):

• Platform: Windows
• Browser: Edge
• Jellyfin Version: 10.9.1

[koyote]

I am having the same issue since updating to 10.9.1 (from 10.8.13) on ubuntu 24.04 with firefox when using direct play.

Not all videos have the issue though, most of them slowly get out of sync as you progress through the video.

[PeachesMLG]

Im also experiencing this, Sometimes it can be as much as 10 seconds ahead.

Restarting my servers and refreshing the cache seems to "fix" it

[Eisaichen]

10.9.2 seems to mitigate this problem with this PR jellyfin/jellyfin#11689, however, it is not completely fixed.
And the audio seems out of sync either.