Ideally first of all I don't want MP3TAG remapping ANY tags I haven't explicitly changed.
Second, I do understand the variation in handling TDRL between both apps. I understood the way I'm using RELEASETIME in foobar is actually a custom tag. I did it that way originally because I thought it would be more compatible with other apps (like MP3TAG).
I use tag panel to display field RELEASETIME and it shows up fine on imported files, I don't understand why it is subsequently being rewritten as a different field.
I also thought that the Mapping option should be a way I could deal with that?
If you have a custom tag that has the same name as the tag name for a standard ID3v2 frame - then it will always be changed to that standard frame.
The mapping option cannot change that.