Just got back from a ski trip that encompassed several hundred km of driving and seventeen driving episodes of various lengths (on/off ferries, supercharger stops, short stops, three overnights, etc.) Not once did any of the AP2 functionality return. The ultrasonics are still there for parking, but TACC and AutoSteer are continuously offline now (I saw no collision warnings at all either, worryingly).
As someone who used to lead a software organization, and wrote and debugged a lot of code myself, this smells like a fundamental structural (not design/functional) bug in the AP2 firmware. It could be a memory leak or memory fragmentation that gradually develops over time, causing intermittent failures at first, followed by a complete lock-up (like my car).
I am also wondering if there might be an interaction with TeslaFi also. I have it implemented on my car, and have not successfully tuned it to get the car to sleep. So my car has not been in deep sleep for weeks. I suspect that the "recovery" some people have seen might be due to an implicit cold start of the AP system after a sleep state. That might have temporarily overcome the firmware weakness of 8.1, whereas builds of 8.0 did not have this weakness, so cold starts had no perceptible restorative benefit.
Just a hypothesis...