What's controlling the on/off status of the screen when you are on a call are these settings on the 'Advanced' tab in Settings-System-Power.
Turn off backlight if device is not used for: {select time]
Turn off device if not used for: {select time}
Apparently, talking on the phone is not 'using the device'. The TP touching your ear or cheek does seem to be 'using the device'.
These settings control things regardless of 'Auto adjust backlight'.
The phone will turn back on when the call is ended.
I imagine tweaks could override those settings.
(I'm still trying to make sense of this device
- I now hold the TP away from my ear when talking because I kept opening notes or contacts or ending the call)