If the proximity sensor is covered, and I make a call (via bluetooth headset (double click for last dialed)) or receive a call, the proximity sensor goes to the uncovered state. It then stays in that state until I uncover and then re-cover it. How I checked this was using Elixir 2 and checking the sensor data. I also set up variables in Tasker and Automagic.
So, this was my test procedure:
I start the sensor data in Elixir 2 for the proximity sensor. I cover the sensor with my finger (it now shows "near"). I use my bluetooth headset to place a call. The screen comes on with the current call. I back out of the current screen (the phone screen) and get back to Elixir 2 sensor data. The sensor now shows "far". From that point on, until I uncover the proximity sensor and then cover it, it shows far. I set up variables in Tasker and Automagic that show the same results.
I am running the stock debloated ROM. Rooted and S-Off.