There ist not much to add to Ohrenkinos post.
Don't use the windows explorer to add tags and don't rely on the explorer to view your tags.
I even experienced that after adding a selfdefined tagfield in Mp3Tag and after that editing a tagfield in the explorer, the selfdefined tag got corrupted.
There are many examples that Microsoft has always taken the position that, as the market leader, they don't have to worry about official and unofficial standards.
MP3Tag writes Mood as the official TMOO-frame. It's a ID3v2.4 frame.
The windows explorer writes this field to a PRIV field. PRIV fields are not supported by Mp3Tag.
You can define a column in Mp3Tag for unknown frames with the value %_id3v2_unknown_frames%
It will show PRIV (WM/Mood).
It ssems to me that the explorer does not show official ID3v2 tags if it uses PRIV-tags for the same.
@ohrenkino
The explorer shows the BPM-Tag.