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

[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...