I have both an iPad 9.7 (2017) and an IPad Pro 2 and have found that the contextual menus are different. Using Refine Mask Tool, there are no output options on the smaller iPad.
On both iPads the brush only refined at approx. 50% and didn't fully mask the adjacent area.
When I am using the Selection Brush and switch to Subtract, the selection keeps deselecting.
Most of my work is compositing and these glitches are very frustrating .