Scenario to duplicate…
Key is D major, instrument is Horn in F, no key sig (if it had a key sig it would be A)
When entering a C, F, or G, they default to being sharp. This is…annoying when entering for an existing chart and also rather surprising. You have to cancel out the accidental each time.
It seems to me that would be expected behavior just as entering a C on the QWERTY keyboard in any instrument would yield a C# if it was sharped in its key signature.
It’s incredibly awkward when entering music from parts, though.
If it annoys you when entering the music, put a key signature change on one staff to neutralize the accidentals. (I.e. set the key signature to F and press Alt-Enter to end the popover, not Enter).
To be honest I can’t really see why it’s awkward - surely if the key is D, most of the Cs Fs and Gs in the horn part will be sharp anyway?