MP3TAG messing up with MP3 comments tag when editing

I know what you said.
But that does not anwer what you did in respect to ...
... Mp3tag and Windows version
... which result you got when you checked problematic files for errors (with the linked tools)
Also, a screendump of the extended tags dialogue would be nice.

Esp. the use of Audacity is not the best way to tag files, see here:

or here:

Perhaps you could supply a file that does not behave the way that you want it.