So this has been happening to me as well, and I was basically at the end of my rope until I figured out a "fix" (not a prevention). When your screen blacks out after a call and won't turn on, slide your finger slowly across the top of the phone (over the camera, speaker and sensor). Viola, screen turns on.
This is obviously an enormous bug, but at least I no longer have to REBOOT my phone after every single phone call.
Reply here with the results for you so we can see if it's a true work-around.
(BTW I also have the low-light screen auto-turn off problem.)