When you get this sort of problem, it is often a problem with the source web site data, in this case, for movies,
www.themoviedb.org, which is maintained by users, so can have errors and problems. It is a good idea to check the web site directly when something doesn't seem to be working.
That site does have a record for the
X-men (2000) movie, but it doesn't show up in the drop down list when you just type "X-men" into the search box, so I suspect there is a problem there. I had to search for "X-men", press enter, and see the full list to see the correct record.
But first, check that the [Media Sub Type] flag for the movie is set to "Movie", so the lookup will work correctly.
Then run the "Get Movie & TV Info..." tool again on the movie, and if necessary change the search criteria to just "X-men" not "X-men (2000)" if that is what you have. You should see the correct result, second down in the list. If you select that, and have the "Use image" and "Use information" check boxes ticked, then click OK and the description doesn't update, come back and tell us.
Note that TheMovieDB has a different image to the one I have for that movie, and may not be the one you want. If so, uncheck the "Use image" check box and just leave the "Use information" check box ticked.
I don't know where bad data came from, but if it was already there when you sourced the movie, and MC selected the "X-Men Production Scrapbook (2000)" as the closest match, which it seems to be doing for me, then the bad data would not be overwritten as the "X-Men Production Scrapbook (2000)" record has no description to save. In that case, MC keeps the existing description. Same with any other tags that already have values in them, for which "X-Men Production Scrapbook (2000)" has no values.