7" Touchscreen backlight only dims (solved)


my touchscreen works fine, I got the rotation properly for the case and touch works as expected, only the backlight can’t be dimmed below a certain point. All the following: DPMS force off, echo 1 > /sys/class/backlight/rpi_backlight/power and echo 0 > /sys/class/backlight/rpi_backlight/brightness only make the screen dim somewhat but it’s still pretty bright.

Running a loop to test the brightness levels made it look like ~75 sets the maximum brightness, everything above does not change a thing and everything below makes it go dimmer as expected. However to turn it off I would have to set the brightness to -180…

Any ideas what the reason could be? Everything else works as expected. And searching just returns topics leading to the above ways of controlling the backlight.

Edit: the control board has version 1.1 and the Raspberry is a Pi 3 Model B 1.2
Edit2: kind of feels like a gate that should be pulled up is floating instead, but which?
Edit3: Attaching external 5V makes it behave properly.