(3840x2160, 150%) has 'Display Settings' maximised. Physical screen 2 (2160x3840, 150%) is the 'main display' showing 'Winspector' maximised. Physical screen 3 (1280x400, 100%) has 'Aquasuite' maximised.
I had a similar problem... I connected a 4K dummy via HDMI. Since then, the overlay had disappeared, so I repositioned it and everything worked fine.
Until the next restart, when the overlay disappeared again. I could only disable it manually via the software and then reactivate it. That brought it back, but it was super annoying.
I was able to solve the problem by setting the scaling on all three monitors to the same setting.
In my case, I had to set the virtual 4K from 150% to 100% so that it would work again without any problems.
Maybe this will help someone else.