Certain tags don't update or can't be removed. For example I've noticed this behavior with the "Artist Website" or "License" tags. Simply removing them and saving shows they are removed in Picard, but when you restart Picard and add the album back in the original values will still be there and no changes will have taken effect. Particularly with "Artist Website", I've noticed that removing the original value doesn't do anything, and if you change the original it will simply add your changes along with the original in a single string. For example if the original value is http://xyz.mistake.com and you change that to http://correct.com, it will make the value "http://www.correct.com/http://xyz.mistake.com"
I've tested this with both 2.5.2 and 2.5.6 with multiple albums/songs. The "Artist Website" and "License" tags are the only ones where I've noticed trouble with updating. Adding a new website tag works, it's only modifying an existing tag that seems to give a problem. Updating a song vs an album has no effect.