Weird, but clearly the Dorico process has not yet exited in fact, and must be hung or spinning on something. Can you duplicate this after a reboot deterministically? If so, I suppose the support people would want a Diagnostic Report file from Dorico.
FWIW on my Windows 10 system if I quite Dorico it immediately goes from the task manager.
This thread may be related, as it seems the process has not exited: