Cannot draw over previous brush strokes in a layer

I've encountered a problem in Photoshop in which when I'm drawing on a layer, Photoshop doesn't let me draw over what I've already drawn.
New brush strokes cannot be drawn over strokes I've already drawn.

I've tried resetting my computer, updating Photoshop to the newest version, resetting the system, preferences, and brushes, but after I think it's fixed, the problem comes back, and it's really frustrating.

I know that it's not some key binding that I'm accidentally pressing, because it happens out of nowhere.

If I can't get this problem fixed, there is no way that I can even use this program because I am unable to even draw on a layer.