Photoshop CC 2019: When I paint over an area with the brush tool to mask an area the image is not being masked completely

I'm trying to mask a CMYK image using the brush tool. When I paint over an area with the brush tool to mask an area the image is not being masked completely. It looks like I have the mask black set to like 85% or that the opacity of the brush is set to 85%. The mask black is set to 100% which should mask the image completely.



This is what it looks like:



I can draw a marquee box and do a fill of black and get the same result:



I have tried using the marquee tool with the "Add Vector Mask" button and and it works correctly. But when I use the brush tool on part of the masked area, still set to 100%, I get this:



Now I can the mask to work with the brush tool if I change the mask colors from 100% Black to 100% on all CMYK colors. Which is counterintuitive.

I've tried turning Legacy Compositing with no change.

Other things of note:

Photoshop ver 20.0.2
OS X ver. 10.13.4

Thanks for any help in advance.