I am using a preview build of Windows 10 (currently build 18361.1) and I seem to have a similar issue (maybe a little different). I have reported it in the Windows Feedback Hub, and I uploaded a video capture on the Hub of me reproducing this error. I have not yet had any responses.
In my situation, Focus Assist activates every time I click the Show Desktop button , OR if I minimize all windows so that only the desktop background is showing. Every time I do this, Focus Assist activates. I can repeat the issue by going to the notification bar and turning off Focus Assist, then clicking the Show Desktop button or opening up a window and then minimizing all windows again.
I have narrowed it down to an option in Focus Assist called "When I'm using an app in full screen mode". If I turn this OFF, then problem no longer occurs. However, you may not have this option as I think it is only currently in the preview version of Windows.
In my situation, Windows thinks that when the desktop background is the only thing on the screen, it thinks it's in "full screen mode" and activates Focus Assist.
For the record, I am using an nVidia RTX 2060 with the latest nVidia drivers (currently 419.35, however I have used previous drivers and had the same issue). I mention the video card and driver as this issue seems to be display/full screen related and I thought there might be a connection.
The below screenshot shows the option I'm talking about. If I disable this option, then the problem goes away, but Windows should not been treating the desktop background as a "full screen" app to begin with. Something is wrong.