Edited my first reply with the ATA v altitude drop off data.
The supercharger in the DB601A was continuous type thing through a fluid coupling (as I recall which means its output was continous up to FTH). At Sea level its doing very little work but as you climb it works progressively harder maintain the ATA. However it can only work so hard eventually it gets to its max output that becomes Full throttle height thats around the 5000m. That is exactly what I am seeing.
The Flight test I did was on the Steppe map Full throttle Prop pitch AUTO max ATA (non WEP). The numbers recorded exactly as shown on the climb. So the indications look good to me. What we dont know is the Power output going to the FM.
Cheesehawk you said:
"After this point, the ATA quickly drops to about 0.8 at 6k, and the prop pitch coarsens to about 1500rpm."
I am not seeing anything remotely like this I have done it 3 times with identical results. I suggest you repeat the flight test exactly as I did it and see what
Attached file is the single mission I was using for the test. Has you starting at 5000m. Test offline in a dedicted test environment. Who knows what has gone on previously On line. .... You might have thrashed your engine getting to 5000m
You also said:
"From my understanding, the AutoPP would adjust the prop based on throttle position, so it shouldn't have been coarsening the pitch when the air pressure dropped."
Its a function of TAS. Thats why in MANUAL or in the E3 in a constant IAS Climb at constant ATA you need to vary the prop to hold the RPM. Above FTH the throttle is firewalled the only variables in the climb are ATA and TAS .