Tablature string misbehaves after re-tuning string.

I had a guitar tablature score in standard tuning. When I changed the tuning of one string (3rd string from G to F#) I could no longer enter a note on it properly.
The first thing I noticed was that I couldn’t use ctrl-up to move an F# from the 4th string, 4th fret, to the new open F# string. When I tried inputting notes on the F# string directly, I couldn’t.
I thought it may have been a problem with my file, so I opened a brand new file. Entered a chord.
Changed the tuning of the 3rd string from G to F#.
Now I can use ctrl-up to shift a note from the 4th to the 3rd string, but i cannot enter any notes on the F# string. All the other strings are fine.
I tried saving the file, closing MS, and reopening. I still cannot edit the 3rd string by using number keys.
I have attached the file.