Grandstream 21xx screen saver / dim

[AlexanderReichert]

I am utterly confused about what I should expect to see when it comes to screen saver / dimming at the Grandstream 21xx phone:

I have two 3cx phone systems all with Grandstreams 2130 and 2160 - they literally all configured the same way:
Blacklight timeout 1 minute
Screensaver timeout 3 minutes
all on the same firmware: 1.0.9.102

but they all behaving different almost looks like at will

1) right after a reboot the blacklight timeout and screensaver seem to be triggered as configured
but
a) some go totally dark with the screen saver
b) some display a dime 3cx logo
c) some display the grandstream screen saver (pretty pictures scrolling through)

2) after a while the blacklight and screen saver is not triggered at all. no voicemail is waiting, no missed call

3) if there is a missed call: looking at that missed call and so to speak setting it "read" will trigger the blacklight timeout and screen saver timeout again. .... for a while until it is stuck again and never triggered.

I don't understand what I should expect to see, what makes it "stuck"; what screen saver should actually be shown?