debian+phosh running on PinePhone

[fpb4]

Boots, stands by a perfectly visible lock screen, unlock, diplays phosh GUI for a couple seconds and dims to a dark screen

Slowly "dims" ?
What's the content of /sys/devices/platform/soc/1c2b000.i2c/i2c-1/1-0048/iio:device1/in_proximity_raw ?
And what happens if you delete file /etc/udev/rules.d/10-proximity.rules ?

Removing the udev rule did the trick - cheers!
Couldn't get in_proximity_raw initial value, but once booted without the rule it's set to 2443.