Setting correct note on lute sub-bass strings?

How do you make the sub-bass string notes in lute pieces appear correctly?

I have this piece in G Minor, where all b notes are flat. I'm trying to arrange it for a baroque lute, meaning all notes that use the 6th sub-bass string (the low b denoted with a "5" in the lute tabs), should also be flat.

However, Musescore doesn't let me make this note flat. It only lets me set that string to b natural or c flat. It's like it's not applying the key to the sub-bass strings.

Attached in my sample piece. You can see an example in first bass note in measure 17. How do I fix this?