HVAC switches to defrost under load.
Here's my other issue on my truck ('01 Dodge Ram 4x4).
Say I have the A/C pointing at my face. If I'm on the freeway and give it 3/4 throttle so the load increases on the engine, the vacuum change causes my HVAC to switch to defrost.
I've checked with tech manuals and it is supposedly a common problem says to replace both check valves on the vac lines near the intake manifold.
I pulled both valves and checked them and they hold vacuum. The lines don't seem to have any leaks either.