Not all key signatures enter in edit mode with double-click
1) Open Musescore and Esc the Start window
2) Insert a new key signature (ex. 3 sharps )
3) double-click on key signature on first staff, it enters in edit-mode -> Good
4) double-click on key signature on second staff, it doesn't enter in edit-mode -> Bad
Of course, when not properly entering in edit mode, you cannot re-position the key signature using mouse click-and-drag, nor using arrow keys.
Using Musescore 3.3.0 in Windows 10.