Hello, fantastic app!
When the challenges get too hard, I can bypass them by pressing the bottom left button that shows all my recent open apps, holding down the PlugBrain challenge window, and dragging it to the top. This lets me keep both the challenge window and the locked app open, sample:

I'm not sure of a great way around this. I've never interacted with this kind of feature in any of my apps - I'm not sure if it could be blocked, or if the app could break out of it and back into full screen mode, etc.
Cheers!