Beginner... but very weird mask behaviour???

I started to try masks but very quickly getting some strange effects. I tried brush, path, draw on a FilmicRGB mask. About the only other tool active is Exposure.
Why are the mask areas “raining down”? Why does their scope depend on the underlying image?
dt 3.1.0 Ubuntu 18.04
Am I doing something daft?!

Does this happen when you have OpenCL disabled?

lower your mask opacity.

“activate openCL support” is shown as “not available” in preferences. That makes sense to me as I have no graphics card. I just use the graphics built into the i7 CPU.

Thanks. (Also I got a bit confused by the two opacity sliders, I’ve referred to the user manual)