The tags may look fine in one program but in reality are not fine. I've had that issue where I've thought the tags where fine and I load the files into Foobar2000 and the tags where not all fine. The only way to make sure your tags are actually fine is to delete them and put them back in. MP3Tag will allow you to do this.
Another thing to try before deleting tags is to remove your Mixtape files and put them back in in groups noting which ones you've put back in in the last group. Then if it starts to act up, you can then try to narrow it down to try to find which file(s) are the issue.