What I mostly use mp3tag for is to change all of the same tag in a set of files (the tracks of a CD).
In 2.65, this worked fine: you construct the, say "artist," tag, select all files and type the new tag into the artist filed on the left side of the screen, and then "save tag." That tag field in every file selected is then changed.
In 2.65a you select all files and as soon as you attempt to type a new tag value into one of the fields in the left panel, all of the files are immediately de-selected.
I played around but do not see any way around this.
I wend back to using 2.65 and it works fine.