Yes, I'm aware of that and I've decided against automagically resolving this. because this would be a breaking change for users that rely on the original semantics of this field.
A way to workaround this is to create a field mapping in Mp3tag at Options > Tags > Mapping. To use GROUPING as the name for the iTunes grouping field in both ID3v2 and MP4, you'd create a new mapping
Tag: MP4
Source: CONTENTGROUP
Target: GROUPING
To use WORK as the name for the iTunes work name in both ID3v2 and MP4, you'd create a new mapping
Tag: ID3v2
Source: CONTENTGROUP
Target: WORK
Let me know whether this works for you.
Kind regards
– Florian