It is working as expected here. In the Key Ed are you possibly trying to insert a Signature change somewhere other than a bar line (it won’t do that anywhere)?
I’m trying to insert a key signature the same way I do in the project window - so Draw tool and then on the bar lines. It works in the project window but doesn’t work in the Global Tracks in the Key Editor. In fact I can’t do anything with the key signature track - so where I’ve created key signatures in the Project window I can’t drag them in Global Tracks.
But then went to check if I could even Select and Delete in order to provide a fuller answer. Once I selected and then deleted a time signature I could now place time signatures as expected- almost as though a lock was removed. Before that I couldn’t do anything. Not sure what happened but thanks for making me go back and try again!