If you select one of the modes for pixel selection (New, Add, Subtract, Intersect) on the display, then use a MacBook Pro Touch Bar to change it, the one on the screen doesn't synchronise with the one on the Touch Bar, and vice versa. The tool you're using seems to synchronise with the tool most recently selected, whether it be on display or Touch Bar.
Am on Affinity Photo 1.8.1.