problem with flattening/merge visible B&W photo's (photoshop CC 2019)

On below screen shot the result of flattening/merging layers before saving.

These red patches occur only in black&white pictures, in colour there are no problems.

The patches look like remnants of masks (same colour). I never had this problem before the latest update off Photoshop!

I'm using a mac pro (2012) with high sierra and I do have a compatible Graphics card.

Does someone know what the problem is and how to solve it?

GPU on/off makes no difference. All suggestions are welcome!