i have been using back tap since 13pro, i have set double tap to screen lock and triple tap to Screenshot, since then i have moved to 15pro and still have same issues with back tap as were in 13pro.
some times it works as intented on Gentle taps, sometimes hitting reasonably hard enough also doesn’t trigger the functions, sometimes gently keep phone on desk triggers triple tap. i haven’t found a consistent way of making it work.
i have tapped on regions above Apple logo and on apple logo. Tapping on corner of camera island works the most of the time but still inconsistent
This is a half bake featured that never got fully implemented, but does occasionally work for those that care to use it. In order for it to work reliably you have to not only have the phone since the taps but also have the phone since the movement forward from the tap so you kinda have to do it hard enough to make the phone physically move forward that quarter of a millimeter.