[CrystalP]

If the same movie becomes the first in the list of versions, we manage artwork for this the same movie with others versions from here :
But the artworks and the list of artworks doesn't seem to display properly. This behaviour is only for the first movie.

You can go to versions manager before the addition of another version and you'll see that the default version art is just a thumbnail, for all movies.
Adding a movie as a version of another copies the movie art to the new version art to preserve any custom art (as the transformation into a version essentially deletes the movie that's being converted), that's why the art of the second version looks more complete.

Now, without changing any version art, if you leave the versions manager screen and play a movie from the library, the selection screen (or the versions folder) will show for the original "Standard Edition" version the movie art in the absence of custom version art (the fallback described earlier)

But if I delete a movie version from the list, I don't expect that this movie to be removed from the library.
I expect that this movie is always in the library

It was removed when it was added as a version of another movie (not just hidden). It's a limitation of the current design.
Best we could do for v21 is explain a bit better in the Remove button popup that you have to update the library to bring it back as an individual entry in the library.