I've been using Photoshop for years but this issue has me totally baffled. It must be some setting I changed, but I have NO IDEA why this happens or how to change it.
The issue is that on layers masks (doesn't matter what kind), the erase tool will ALWAYS leave some percentage of opacity, no matter what I try. You can see the vague grey-ish area that I attempted to erase, but it will never go to full transparancy. See image below.