You know that copy and pasting cover art is not a way I would recommend. The reason why is because it increase the size of the Cover Art significantly,
I've investigated this issue and couldn't find any problems related to Mp3tag.
One important thing to mention is, that if the programs are running under different user IDs (e.g., one as Administrator, one as standard user), copy paste is not working. This is a security measurement of Windows to ensure that one has no access to the clipboard of the other.