SOLVED: Embedded movie cover art correctly extracted from mp4 but not from mkv

[Papero]

I am relatively new to jellyfin, so bear with me... maybe I just have some wrong settings...
I have movies with embedded cover art (cover.jpg attached as metadata) and I have set jellyfin to use "Embedded Image Extractor" as highest priority in the Image fetchers list.
I have noticed that cover arts of mp4 movies are correctly displayed (the image extraction works perfectly) while mkv movies do not seem working.
Any reason for that? I can convert mkv to mp4 with ffmpeg and then re-embed the cover art with mp3tag, but I would rather check if there is something else I should try first.

[TheDreadPirate]

Can you share the output of ffprobe on a MKV and on a working MP4?

[Papero]

I further investigated and discovered the source of the problem, which is not related to the file format.
This is what happened. I had a series of mkv movies with cover art embedded in the metadata but also with a jpg file associated containing the cover.
Basically files such as:
Movie1.jpg
Movie1.mkv
Movie2.jpg
Movie2.mkv
etc.
Since the cover was also embedded I thought: "Oh well... let me delete the jpg files and see if jellyfin can extract the images directly from the video file".
I did so and my directory became:
Movie1.mkv
Movie2.mkv
etc.
Then I rescanned the library and all cover arts disappeared. I interpreted that as if jellyfin was not able to extract the embedded images.
In reality, I believe, jellyfin does not find the jpg files anymore, but having videos already present in the library, is not programmed to refetch the covers from the files.
The solution was removing the mkv files, rescanning the library and then reintroducing the mkv files and rescanning again. At that point all embedded images were correctly recovered from the videos.

My mistake...