Brush faces disappearing?

I’m working on a project for class, but all of a sudden one of my brushes started acting weird. I don’t know what’s wrong.

It would help to solve if you told “what” is behaving wrong there.
I can see 2 brushs, guess the outer one is an additive and the smaller one a subtractive and it doesn’t work the way you want to?

My first try would be to play around with the “order” settings found on each bsp, so set the additive-brush to “first” and the subtractive to “last”.
I encountered some issues with bsp myself when changing geometry etc.
Sometimes it can help to rearrange the order to “reset” the calculation, I guess.