'Hide Stems/Rests' randomly checked!?

Hello folks

Just recently updated from Cubase 9 Pro to 12 Pro.
I mainly use the Score Editor.
I recently tried going briefly going back to C9 to see if it could still open things that had progress recently saved in C12 - and it seems that it can.
But I suddenly noticed the Trombone 1 stave had no stems or rests. Odd.
I checked the Staff in Score Settings and saw that in the Tablature tab - a tab I never use - Hide Stems/Rests was checked. I couldn’t immediately uncheck this as it was greyed out. But first checking Tablature Mode allowed me to uncheck Hide Stems/Rests (and the uncheck Tablature mode again) allowed me to resolve the problem - the Trombone line now had stems and rests. Although for some reason it had changed to the Treble Clef, but that’s easily sorted.
Weird though!
I did not save the project at this point, but shut down C9 and reopened the same save in C12. The Trombone line had stems and rests as they should, but then I checked in the Tablature tab and saw that Hide Stems/Rests was checked! Then I saw that various other staves randomly had Hide Stems/Rests checked - even though in C12 everything displayed properly!

What could be going on here? I never touch the Tablature tab. Have I done something silly here, or is this a bug?