Perhaps this helps:
When I press Ctrl-N and a field in the tag panel was originally selected in toto, then, after pressing Ctrl-N the write cursor moves to the front of the field without selection.
When you then switch to a different application and return to MP3tag (give it the focus again), then the whole field in the tag panel is selected. So it may be a little hard to spot where the input focus is.
(This was also tested under W10)