I've found a bug in MP3Tag v2.39 (and it remains in v2.39a) export.
If I put a $put or $puts command in the second or third place (in place of 'do if true', 'do if false') in an $if statement, the command will be executed independently of my conditions.
Here is a stupid example, run it some audio files having fields ARTIST and TRACK:
$if(%artist%,Artist OK,$put(ctr,$num(%track%,2) no artist))
%artist% - $get(ctr)
As you can see, the variable ctr should be nothing if the audio file has the field ARTIST. BUT it changes in every iteration.