Upgrading to 1903 broke this for me. Have an NVIDIA GeForce RTX 2070 Max-Q and Windows thinks the desktop is a full screen app and enables Focus Assist for all profiles.
There is a possible workaround for this. If you have GeForce Experience installed, open it up and disable "in-game overlay" in the General settings. When I do this, it stops Windows 10 from thinking the desktop is 'full screen' mode.
Myself and many others have reported this problem in the Microsoft Feedback Hub.
Thanks, I found this out a few hours ago. I also submitted feedback. Found that when I created a new profile it worked fine until I logged into GeForce Experience. That at least pointed me in the right direction knowing that application was the root cause.