If I understand correctly, there is a common issue.
Since now we can have rename masks for normal files and a different one for compilations, you need to make sure both of your masks are filled in, even if they are the same mask.
I haven’t tried this but if Musicbrainz comes back with Is Compilation? checked, then it will try to use the compilation mask and if there isn’t one, it doesn’t rename the file. This is why some people were getting some files renamed but not others.
If this is indeed what is happening, maybe there could be a condition in the save method that defaults to the main rename mask? I’m not sure what would be more correct : either not rename it or rename with the normal mask.