Originally Posted by DrDan
Is you BIAB track frozen? Otherwise, I would say no to that question. Can you send us the midi track to look at it.

First, no it wasn't frozen.
Out of desperation, I muted all of the tracks except the one in question, then exported the file as a MIDI, having just the one track I needed, and then imported the file into the Melody track and edited.

Afterwards, I exited the original MGU file and opened it again and I was able to edit the track in question.
So, still a mystery to me what caused the issue. But now I know what to do, should it happen again!

Thanks, sorry for my ineptitude!