Subtract from mask with brush not removing 100%

Using Lightroom Classic 11.311 on Windows.

I have a photo of a boy and using Create New Mask > Select Subject to mask him. However it's not only the boy that's selected, so I need to remove some of the mask. But it's almost impossible.

I go to Subtract > Brush and set the settings to

Size 10
Feather 0

Flow 1

Density 100

But it's like the density is 10, not 100. The mask is only partly removed. See red circle of screenshot

How may I fully subtract from the Subject-mask?