Mask tool doesn't work properly after updates

I have an issue that persists in both Photoshop apps - in the classic one and in Beta one, after recent updates. When I mask an object and then try to erase a part of it on the mask, a slight trace of the object always remains! I am unable to erase the object at 100% of opacity. It leaves a trace of it anyways, like 10% of the object opacity. Who else has the same issue? Any solutions found?

Thanks a lot!

Poly