Unusual subtitle option behaviour - unexpected "Secondary Subtitles" option appears from nowhere

[TimF0]

This issue respects the following points:

• This is a bug, not a question or a configuration issue; Please visit our forum or chat rooms first to troubleshoot with volunteers, before creating a report. The links can be found here.
• This issue is not already reported on GitHub (I've searched it).
• I'm using an up to date version of Jellyfin Server stable, unstable or master; We generally do not support previous older versions. If possible, please update to the latest version before opening an issue.
• I agree to follow Jellyfin's Code of Conduct.
• This report addresses only a single issue; If you encounter multiple issues, kindly create separate reports for each one.

Description of the bug

I have a film (mkv format) with no embedded subtitles. I've downloaded an .srt subtitle file, renamed it appropriately and added it to the server. If I play the film, I get an "English - SUBRIP - External" option showing up - so far, so good. However, if I try and turn off the subtitles part way through the film, I not only get "Off" and "English - SUBRIP - External" as options, but also a "Secondary Subtitles" option which has appeared from (seemingly) nowhere.

Reproduction steps

1. Download a file with no embedded subtitles
2. Download a suitable .srt file
3. Rename the subtitle file to (film file name).eng.srt and add to the film directory
4. Refresh the metadata (I found this was necessary to get it to pick up the subtitle file)
5. Start the film with no subtitles - note the 'CC' button is displayed, and clicking it gives the "Off" option (already selected), and an "English - SUBRIP - External" option.
6. Click the "English - SUBRIP" option - the film continues to play but shows the subtitles
7. Click the 'CC' button again - this time, the options displayed are "Secondary Subtitles", "Off" and "English - SUBRIP - External" (which is selected)

What is the current bug behavior?

An extra "Secondary Subtitles" option is displayed in addition to the existing "SUBRIP" option

What is the expected correct behavior?

With only one set of subtitles, only one option other than "Off" should be selectable.

Jellyfin Server version

10.10.0+

Specify commit id

No response

Specify unstable release number

No response

Specify version number

No response

Specify the build version

10.10.3

Environment

- OS: Unraid 6.12.13
- Linux Kernel: (whatever 6.12.13 is built on)
- Virtualization: none
- Clients: Browser
- Browser: Firefox 132.0.2
- FFmpeg Version: 7.0.2-Jellyfin
- Playback Method: Direct Play (tested on my phone and also happens on HTML Video player)
- Hardware Acceleration: NVENC
- GPU Model: GTX 1050Ti
- Plugins: AudioDB, MusicBrainz, OMDb, Studio Images, TMDb
- Reverse Proxy: none
- Base URL: none
- Networking:
- Storage: local

Jellyfin logs

[2024-11-25 17:03:47.182 +00:00] [INF] [45] Jellyfin.Api.Helpers.MediaInfoHelper: User policy for "RaveDrake". EnablePlaybackRemuxing: True EnableVideoPlaybackTranscoding: True EnableAudioPlaybackTranscoding: True
[2024-11-25 17:03:47.492 +00:00] [INF] [52] Jellyfin.Api.Controllers.DynamicHlsController: Current HLS implementation doesn't support non-keyframe breaks but one is requested, ignoring that request
[2024-11-25 17:03:47.492 +00:00] [INF] [52] MediaBrowser.MediaEncoding.Transcoding.TranscodeManager: "/usr/lib/jellyfin-ffmpeg/ffmpeg" "-analyzeduration 200M -probesize 1G -fflags +genpts -f matroska  -i file:\"/media/animated_films/Despicable Me/Despicable Me 2 (2013) 1080p.mkv\" -map_metadata -1 -map_chapters -1 -threads 0 -map 0:0 -map 0:1 -map -0:s -codec:v:0 copy -bsf:v h264_mp4toannexb -start_at_zero -codec:a:0 libfdk_aac -ac 2 -ab 256000 -af \"volume=2\" -copyts -avoid_negative_ts disabled -max_muxing_queue_size 2048 -f hls -max_delay 5000000 -hls_time 6 -hls_segment_type fmp4 -hls_fmp4_init_filename \"e19d382e41e89e5a0e05e45a342d6564-1.mp4\" -start_number 0 -hls_segment_filename \"/tmp/e19d382e41e89e5a0e05e45a342d6564%d.mp4\" -hls_playlist_type vod -hls_list_size 0 -y \"/tmp/e19d382e41e89e5a0e05e45a342d6564.m3u8\""
[2024-11-25 17:04:07.823 +00:00] [INF] [51] MediaBrowser.Controller.MediaEncoding.TranscodingJob: Stopping ffmpeg process with q command for "/tmp/e19d382e41e89e5a0e05e45a342d6564.m3u8"
[2024-11-25 17:04:08.060 +00:00] [INF] [51] MediaBrowser.MediaEncoding.Transcoding.TranscodeManager: FFmpeg exited with code 0
[2024-11-25 17:04:08.060 +00:00] [INF] [51] MediaBrowser.MediaEncoding.Transcoding.TranscodeManager: Deleting partial stream file(s) "/tmp/e19d382e41e89e5a0e05e45a342d6564.m3u8"
[2024-11-25 17:04:09.904 +00:00] [INF] [51] Emby.Server.Implementations.Session.SessionManager: Playback stopped reported by app "Jellyfin Web" "10.10.3" playing "Despicable Me 2". Stopped at "17309" ms

FFmpeg logs

Client / Browser logs

No response

Relevant screenshots or videos

Options available when subtitles are currently off:

Options available once subtitles are turned on:

Additional information

As mentioned above, this also happens if I play the film on my phone.