What is the sudden change in the exhaust plume at occurs here [1] at 149km / 17650 kph? I believe that's squarely in the thermosphere, and there aren't any step changes in temperature/pressure at that point that I'm aware of. It would make sense if that were near the mesopause, but that's around 100km.
Maybe it's something on SpaceX's side of things, a change in the burn profile? Perhaps it's related to losing the Starship about a minute later.