it seems to me that APETAGEX (APEv2 tags) are not correctly processed by Mp3tag.
As you can see in attachment 2, the "Erweiterte Tags..." menu function does only show 4 tags, while the first attachment (hex dump of mp3 file tail) shows other tags (ALBUM, COMMENT). Specifically, a cover jpg is stored as of offset 005E:FABA (see black part) in the mp3 file, but is not detected nor displayed by Mp3tag.