I don't know a thing about newer cars, but I'll guess.

Sounds like the vacuum dashpots are sensing low vacuum from your manifold and reverting to the 'fail safe' mode of Defrost when vacuum is low.

That said, I'll guess there is a vacuum accumulator that is intended to sustain vacuum for the HVAC system during full-throttle low-manifold vacuum conditions, and the accumulator or a dashpot has a leak, giving you the results you report.

In short, a vacuum leak in the HVAC tubing is where I would be looking.

I will delete this post when someone who knows this car better can set you straight, since I'm only guessing.

Best Regards,
Jim