Again, thank you for helping out.
Yup, looks like mp3tag isn't the issue. The thing is, I would use Windows Media Player (Version 12.0.7601.17514) to check out the files. And under Tools/Options/Library(tab)/Automatic media information updates for files, these were check-boxed:
(x) Retrieve additional information from the Internet
(x) Only add missing information.
Which may have played a role. I've also read that WMP may have issues with ID3 tags (but don't know enough about this topic). In any event, disabling that feature resulted in at least one persistent problem: Incorrect display of cover art. It would tend to get stuck on one image as one went through a playlist.
That is not a problem if I use Kantaris media player. All tags are intact and the cover art is correct.
Is it the general consensus that Windows Media Player is a problem? I can deal with it, but some less techicaly-minded friends only use WMP and I guess they'll experience cover art display problems (at a minimum).