Trying to use layer mask in PS2021. Opacity, Flow and Density all set to 100. Blend Mode set to Normal. Foreground color set to black. When I paint, it comes out at 50% opacity, at best. Using an iMac
I don't see anything in the screenshot that would cause the problem. In the screenshot the layer thumbnail is selected; the mask is not selected, but that may just be a coincidense. Are you by any chance using a brush that is less than 100% opacity? If so, try selecting one of the hard edge brushes in the General Brushes folder.