Delete first beat of first measure invalidates key signature
I'm not sure I have the detail right on what the trigger, but these steps fail every time:
1) new score, treble staff, key of B (anything but C), 4/4 time
2) enter four quarter rests into first measure
3) select the first rest
5) enter notes anywhere else, via computer keyboard or mouse
Result: pitches are entered as if the key is C. That is, type an A you see an A and hear an A, but it should be A# according to the key signature. Same for all other notes affected by the key. Press "Up" and a sharp sign is added, thus showing it really is acting like there is no key signature, even though one is displayed on the first system. Look more closely, you'll see no key sig on subsequent systems, though.
So it seems the keysig map has been compromised.