I'm unable to reproduce this.
This is my exact test case, created with a new Database
1) Added one album, mp3, no previous pictures attached
2) Opened the albums files in the Advanced tag editor
3) Added an attached picture (jpg), set type to Cover (front) and description to Cover (front)
4) Locked the control and pressed Apply to all >>
5) Inspected one file via Properties (one APIC was added)
6) Opened the album again with the Advanced tag editor
7) Only one pic was shown
8) Pressed Apply to all >>
9) Opened a file from Properties again, only one APIC
So, I could not reproduce this.
Sorry, when I mentioned file-type I was talking about MP3, FLAC etc.
Picture type: Cover Art (Front) and/or Cover Art (Back)
Which picture type, descriptions and file types are you using?