Since playing around a bit with the Notch Bar support, I've been having a persistent problem with the screen resolution. I am not aware that I could have made any other changes.
I have now completely disabled the Notch Bar feature also deleted all actions... nevertheless BTT is shrinking my screen when I use the snap area functionality causing a black bar to appear at the edges.
btt contains no code to change screen resolutions, maybe some other app?
No, its definitive BTT.
It starts when snapping a window (I have no other app with this feature installed) and recover the old resolution when I quit BTT. I never happens when I am not running BTT.
But maybe its not a real change in resolution but a kind of shrink/zoom.
In the end a black bar in the size of the notch bar appears left, right and at the top.
To be on the safe side, I had deactivated BTT for the last 30 days and have not observed this effect once since then. Since I have now re-enabled BTT, the problem immediately reappears. I really need help here or I can't use BTT anymore unfortunately 
BTT definitely doesn't contain code that can change the screen resolution.
Maybe there is some keyboard shortcut you are triggering with BTT that can do this? (I'm not aware of any though).
Or maybe some script you are running with BTT? Are you using a self made setup in BTT or one created by somebody else?
As mentioned, it's not really a change in resolution, but a sort of downsizing effect. You can see it in the fact that only Mission Control continues to use the full screen. Everything is done seamlessly and smoothly, just like the screen zoom function of MacOS.
The Macbook is new and mostly clean. I am currently using BTT even without any scripts or actions - I only use the Window Snap function so far.
The problems started noticeably when I experimented with the new notch bar support (but I have since disabled all of it again). The notch feature also doesn't work for me as it should, so I suspect the problems are related to that as well.
The effect itself kicks in when I drag a window to one of the corners and BTT displays the rectangle to snap the window.
There is no other automation or tool running and especially nothing that could handle the window movement.
If I quit BTT in the zoomed out state the window zooms back to full size, so again BTT has a direct effect.
I have now reinstalled BTT and the problem was initially gone - however, I now have various other effects such as the BTT's macOS menu items are dead and not clickable (so I could not activate my license). At one point I could even see that the LEFT area next to the notch was overlaid with a gray haze (this could be an indication why the items were not clickable).
After some "back and forth clicking" the gray haze disappeared, but at that moment the old zoom out effect kicked in again.
If you're interessted I'm happy to present it to you live - some of the effects might be quite confusing to describe.