The bug is still here. At least now I now exactly what happened.
I imported a few movies, after I finished importing I select them all and use TITLE CLEANING & RENAMING.
After multiple changes were over, first movie was overwriten with the last movie. First and the last movies I selected for "title cleaning" now had the same data and the same cover.
I'm not sure... but it might be. I done cleaning titles often, usually right before or after web import, so previously I assumed, that this is done by importing...
Of course, it doesn't happen always! So, there is still a question when or why.