nope AP is designed spookily. shit that works one day makes no sense the next bc something they don't tell you about is microscopically different, and there is nothing intuitive about the UI. it's the most user-hostile program i've literally ever used.
The solution, it turns out, is you have to unclick "protect alpha" in the contextual menu for the mask layer to be able to paint it in and out. not sure if that's what you're talking about bc everything is ridiculously complicated and opaque in AP.
Even the way layers, masks, and adjustments are supposed to be stacked in order to have the desired effect is the opposite of intuitive - they don't appear in the order they affect each other, and then they affect each other in spooky and unwanted ways.
have never had a consistently predictable experience using AP from one session to the next. also the most haunted program i've ever used.
dear affinity programmers: please fix everything about this program. it is not at all intuitive, it asks way too much of the user, unless the user is a programmer with rectilinear thinking who learns by rote and memorises technical manuals with ease. The rest of us are sturrugguling!