Originally Posted by eclyptos
Try again without "Adapt to lighting conditions" in Display > Brightness. And when the screen go off in the middle of the call scroll the phone. If the screen show up keep it in that way. If not try disabilitate also Optimized Backlight. Restart the Phone. Sometime happen with Android.
Thanks for the suggestion, but no luck.
I found what the issue was - under Accessibility options, there is an option Power button ends call. That was enabled.
I don't remember enabling that option and I've never had the need to enter that menu.