Painting on my mask in white is making white marks on my image

[mayflyaway]

This is going to sound crazy and I feel like i am going insane.

I am not new to photoshop - i have been making all kinds of masks and composites for over a decade and have never seen this happen. I am masking a horse in an image in order to replace the arena background with a solid color - but - as I am painting in white on my image (to make the parts i want to show through) it is leaving faint white marks on the actual horse image. If I set the background fill color layer to dark green and paint on the horse part of the mask, it leaves faint dark green stripes on the horse. Please note, I am definitely clicked on the mask itself and NOT on the image. And I definitely have my paintbrush selected everything at 100% and the color is white. It *should* simply be masking the horse like usual. Like, I've literally done this exact same thing 8 times in the last 2 days without issue.

It started doing this in Adobe CC 2019, so I went ahead and updated to CC 2020. I reopened the image in PS CC 2020 and it is still doing the same thing. I'm utterly baffled. Help?

Any clues?

In the video I paint on the layer mask, then hit ctrl-z to undo, at which point the difference is pretty visible.

[Trevor.Dennis]

I'm struggling to see the problem with the video.  That is to say, I can't see any artefacts being left on the horse.  From what we can see you are painting with white onto the layer mask over the area of the horse, which is being revealed as we'd expect.  It looks like you have the normal paint brush selected, and that its flow and opacity are 100% and its blend mode is normal.

Are there any brush settings that might be causing an issue.  It looks like a fully soft round brush with no pressure settings turned on.  Is there anything going on in the brush settinghs panel?

Are the underlying black and white layers both set to normal?

Try Alt (Opt) clicking the layer mask to display it full screen.

Try making a selection around the artefact area, and Alt (Opt) backspace to fill with white. 

Try disabling the layer mask.

Is it a display issue, as in video card driver, or one of those issues that checking 'Legacy compositing' in Preferences > Performance fixes.  I have not had to use that for a couple of version changes myself.

Any other info you can give?