However, lately the part where I've painted on stays at about 7% visible, no matter how many times I paint over it, kind of like there's some setting which makes layer masks only work to a certain percentage, then doesn't let you to erase anymore.
***THE BRUSH OPACITY IS AT 100%.*** This happens on all the files I've tested on (including new ones). Both of the layers blend modes are normal.
Is this a glitch or a setting I added then forgot about? Even after I restarted the computer, the problem persisted, so I'm guessing it's a setting, but which one?
Sounds like a glitch. A perfectly black layer mask should make the layer perfectly transparent, and that's what I see it do.
Is your document an RGB document?
Can you do some screen grabs showing the problem graphically, and post them here? Make sure your Layers panel is showing.
Just to add, when using masks it easy to set the foreground and background colors to pure black and pure white and vice versa by hitting the D key for foreground black and background white
and the X key to switch to foreground white and background black.