This issue started happening yesterday, alas!
There have been many threads popping up on the forums mentioning it.
We are trying to keep everything in the same thread:
https://forums.flightsimulator.com/t/oat-of-248-c-at-fl285-impacts-negatively-on-aircraft-performance-and-autopilot-function/478029
Please continue, if you wish, in that thread.
For now, closing this as a duplicate.