Windows Defender in Sandbox

Win10 Pro 1909 18363.535

I see the question has been asked at least twice without answer, a few months ago, but trying again just in case there is now a solution.

Defender is enabled for Sandbox (setx /M MP_FORCE_USE_SANDBOX 1. Got "SUCCESS: Specified value was saved.")
Task Manager Details shows MsMpEng.exe and MsMpEngCP.exe running.
Run Sandbox as Administrator.
The moment I paste a file onto the desktop I get a message "Turn on virus protection." If I try, I get "PAGE NOT AVAILABLE. Your IT administrator has limited access to some ares of this app...."

When I click OK, I get green ticks for all aspects of "Security at a glance."

But Windows does not detect a virus. This makes Sandbox useless.

EDIT: Tested after signing in as administrator. Same thing. Going to try a Repair Install, see what happens.

EDIT: A repair install did not fix the problem. One of those MS bugs, Defender works, but not in Sandbox.