Perhaps I am having the same issue. Just purchased Affinity Photo 1.8.6 and following a course provided called "10 steps to amazing photos".
When using the Brightness and Contrast adjustment layer, and then inverting it to target a specific area of photo using Black brush it works fine, BUT if I switch to the WHITE brush to correct an overpaint it still
paints as if brush was black. Followed the course almost frame by frame to assure I was following directions...still does not work.
I attached image below and was trying to erase the overpainted water to the paddlers left but to no avail.
Is it me or is there a bug????