Stripes in images created with pixelmath

I found some strange stripes in images I created with pixelmath. I have three images H, S and O. To reproduce I use a very simple math
R: ~O*H
G: S
B: O
This gives me an image consisting of two parts, as can be seen in the screenshot included.

The resulting image is OK only in the upper part:

With more complicated pixel math I am sometimes getting more stripes than two.
I dont think that this is as it is supposed to be.
I am using the latest git version of Siril as of today.

Hello again, I am afraid those stripes might be due to malfunctioning of my graphic card. Not sure yet, I am still investigating.