I can confirm the artwork issue is fixed.
As far as file duration, I performed Copy&Paste tags from one file to another with different duration three times.
There was no problem with the first set of two files, however the bug described above was present in two other sets
I didn't make a backup of the files, so I can't show you the original versions.
However, when I was going to PM the four buggy files to You to fix the problem I copied them into the desktop. Then I cleared the MB now playing list and started to play the files from the desktop to make sure the bug exists. Surprisingly, when I play the files from the different location (i.e. the desktop), the file duration is correct
Media Monkey 4 shows the correct time of the four tracks too (no matter the location).
So I'm sure this bug has nothing in common with tags, BUT I can confirm that the duration of two tracks is shown wrongly in MB after I copied & pasted tags from one file to another.