The only reason I know that a movie would get the correct actors and directors in the metadata fields, but not other fields such as Description, Name, and maybe Cover Art, is that those tags in the file already had a value in them when they were imported.
Auto Import won't overwrite existing tags when it runs automatically. It uses existing tag value ahead on metadata parsed from the filename. Also, running Auto Import on multiple files won't overwrite existing tags. It is only when you run "Get Movie & TV Info..." manually on individual files that you get the option to overwrite existing tags.
I think the "Get Movie & TV Info..." function does use the [Year] if CARNAC can parse it from the filename as part of the Auto Import process. But there were a bunch of changes around that area functionality some time back, and I don't recall how it all ended up.
Search the forum for posts by Hendrik about it. I think he gave some definitive comments.
The only way to test is to have a clean file with no tag values, so that just what is parsed from the filename is used for the lookup. Testing seems to be made more difficult by Windows 10, which doesn't seem to allow the Title tag to be deleted from files anymore. It complains of permissions for a couple of test files I created from existing files. Grrrr. So I'll leave testing to you BigMuscles!