I've just released Mp3tag v3.32d with changes on how Mp3tag writes ID3v2 and WMA tags.
When writing ID3v2 tags, Mp3tag now attempts to preserve the original ID3v2 frames if they are not modified by user input. This means that multiple WXXX frames, even those with identical descriptions, are now simply carried over during rewrites.
In addition, it is now possible to add multiple WXXX frames, including those with the same description, via Mp3tag. This does not conform to the ID3v2 standard, but I believe it is a practical tradeoff that reflects the current reality of ID3v2 tagging.
Because ASF/WMA tagging is very strict, I decided to store multiple values for fields that do not support them as Value 1; Value 2 when writing.
I hope this addresses all issues raised in this topic. Please also note that these changes required significant modifications to the code that handles ID3v2 writing, so please use with caution.