it looks like you have a "Transform" effect on the adjustment layer, and you have 2 keyframes on the scale parameter, so try to turn off your adjustment layer or the transform effect Draw your mask then turn on the adjustment layer or the effects again.
@OussK 's diagnosis is spot on. You are making a mess with your adjustment layer and transforms. This is a typical beginner error of trying to do too much at once in a single comp. Learn to structure your work by isolating steps in pre-comps and educate yourself on how AE actually handles things in a specific order.