FWIW this is a known security bug at Apple. I filed a bug about similar behavior where you can see the desktop briefly without logging in. Apple marked it as a duplicate. https://imgur.com/YxXtU2y

Here are the steps to reproduce:

- Start Mac

- Login

- Turn on Screen Lock: System Preferences > Security > General > Check "Require Password" and Select 5 Seconds.

- Turn on Hot Corner Sleep Display: System Preferences > Mission Control > Hot Corners > Select upper left > Put Display to Sleep > Ok

- Attach external monitor

- Activate hot corner by dragging mouse to upper left corner of screen

- Wait 6 seconds

- Click the mouse to trigger waking the screen

- See brief flash of the desktop without logging in!