This afternoon I noticed something weird. When I had Mp3tag load all of my (to mp3) ripped cd's about one 3rd showed empty title and artist tags (those are the only 2 tags I use). After checking with WinAmp I noticed that the files that did not seem to have contents in those tags actually did have an ID3v2 tag with the artist and title in it.
So I kept looking and I found the cause of this weird issue. I've set Mp3tag to read all tags but only save ID3v2 and APEv2 tags. The files that appeared "tag-less" in Mp3tag did have the artist and title stored in the ID3v2 tag but not in the APE tag.
Apparently, because of the empty APE-tags, Mp3tag does not show the contents of the artist/title tags even though they are available in the ID3v2 tag that also is being read.
The solution is simple though: disable read for APE-tags, open the folders with the mp3's that didn't seem to have tags, they show up now after disabling the read APE-tag setting. After that I've selected all files and hit save so now the APE-tag also contains the artist and title info, just like the rest of the files.
Since I don't think this should happen I'd like to report it as a bug.