Mask in After Effects v 2020 is not accurate
Issues like these usually arise when you either have the layer transformed in 3D-space, or when using distortions and transform-effects on the layer. Since you do not have the 3D-checkbox ticked, I assume that you have some sort of transformation applied in the effect-controls. Did you use something like cc-lens, mesh-warp or turbulent displace? If so, first mask the layer, then precompose it, then apply the warping to it.