FP sensor - what is the rule for displaying

[pio_zen]

It seems that FP indicator should appear white in light environment and grey in dark. But it is completely random.
Can someone from Asus explain what is the rule? And why it doesn't work?
PS. FP sensor still sucks and it looks like hardware issue. I have been using OnePlus 7 (2019...) even with thick protective glass and it was flawless. No matter if display was dirty or not. If FP sensor require completely clean glass than it is design flaw - how it can be clean with touchscreen?!