When working on my iPad, I've noticed that when I use the select subject masking tool and edit the selection it doesn't stay edited when I hit done. It reverts back to the editing with the rest of the photo.
It works for me. Are you sure it isn't working? Try making a really drastic edit with the mask selected. Take all the color out of the subject so it is black and white. Then tap done in the lower right corner. Can you still see the edit on the image? Is the subject still black and white? You won't see it in the edits you made in the edit panel because that returns to the global edits.
Is it possible you have tapped on adjustment slider bars vs dragging the handles for adjustment sliders? We're investigating a known bug related to tapping sliders to "nudge" adjustments = may not see edits saved. Dragging sliders does save edits.