Proximity Sensor is mocked to 5cm, screen still goes dark
Well, I thought I had tried all possible combinations, but seems like I forgot to save the mocked values. With LTR579 ALSPS: 0cm and LTR579 ALSPS-Non Wakeup Secondary: 5cm, it now works as I want it to. I have no idea why the sensor testing app gave me wrong values.
EDIT: Turns out that the app i used to calibrate the sensor with at the very beginning was the reason for the unwated behaviour after all. Just uninstalling it was not enough, I had to reinstall it, choose the option to reset the calibration, uninstall it, reboot the phone, and THEN override the sensors in the XPosed app.