As per the thread title and screen shot, it seems like I cannot set the ungarrison hotkey for fortified churches.
Seems like I am forced to use a different hotkey compared to my other buildings….
I stumbled upon the same problem. It kinda makes no sense, but you can find it under the Hotkey Group/Unit Commands/back to work (mine is German, so I guess its called like that).
That hotkey is in the “Unit Commands”, the “Go Back to Work (Fortified Church, Dock)”.