You are correct that File Explorer tags/metadata doesn't show up in extended tags within Mp3tag. The copyright field in Mp3tag doesn't seem to correspond with the copyright field in File Explorer. This is kind of an annoying issue which should be relatively straightforward (but for some reason isn't).
MetaX looks like one possible solution, however that solution is gonna cost you a little bit of cash (also somewhat annoying).
There MUST be a workaround for this issue, or another piece of software that can deal with File Explorer copyright metadata.
Important: You CAN NOT modify this COPYRIGHT tag in windows 10 file explorer.
Confusingly, Mp3tag can read this copyright tag and show it like this: revealing, that MetaX seems to write 2 additional ITUNES tags secretly. This seems to be caused when you choose "Movie" after opening a Mp4 file in MetaX. Even if you choose "none", you get ITUNESHDVIDEO with value 0 as additional tag.
If you try to modify this COPYRIGHT tag in Mp3tag, you can see it in Mp3tag only, but not in windows 10 file explorer!
On the other hand, MetaX can read this changed value:
In my test, just pressing "Write Queue" in MetaX was not enough to save the changed value back in the Mp4. I had to change at least one character to force MetaX to write it back - and make it visible in windows 10 file explorer:
That's all very inconsistent and confusing.
Maybe @phw can have a look at such a file, knowing now that Exiftool shows a COPYRIGHT tag originally written with MetaX (and visible in windows 10 file explorer) like this:
ExifTool -g7 option:
