Changing Key Signature For One Staff Doesn't Always Work
Holding ctrl and dragging a key signature from the palette onto just one staff line doesn't usually work like it should. It almost always changes the key signature for the entire score. The only option here is to make the others invisible.
Comments
It sure usually does work. If it doesn't for you, please share the score
Drag and drop is always a kind of imprecise and slow method of doing things, lots of room for error. Try simply Ctrl+clicking the key signature (after selecting the proper measure of the proper staff of course).
In reply to Drag and drop is always a… by Marc Sabatella
For the times where it acts bizarre and changes the key signature on some of the surrounding instruments, I found I can just make the ones I don't want seen invisible.