V22.5.1 100% flow and opacity brush on mask still leaves background showing through.

Photoshop v22.5.1

Windows 10 

Using Wacom tablet.

While masking in a new layer with the white brush, even if I set it to 100% opacity and 100% flow it still leaves the layer below slightly visible. No amount of pressure on pen or number of times gets rid of it. Then if I show the quick mask ("\") and paint over with white it goes away. But of course I shouldn't have to show the quick mask for this to work. Thanks