I just found this thread. I've been having this battery capping issue for months now, and I started noticing it right after the summer break because the route for my son's school was always the same every morning. He started going back to school, and my charging frequency started going up and my range down. Less range and it felt like less power too. My route was always the same for the past 2 years, and I knew something was off.
I never imagined Tesla would cap the battery though. I'm on a P85+, I just had a 90% charge, and I drove conservatively @60mph freeway with slow acceleration, to test if it was my driving was causing the problem. I got 162.8 miles with 8% left with an average of 307 Wh/mi.
Man, this is pretty frustrating to hear that Tesla was the root cause. I've added a NHTSA complaint, and hopefully Tesla will make it right.