Further to my last ...
Here’s a scenario...
Let’s assume your boost sensor is reading higher than it should (faulty sensor) or the pressure IS higher than it should be (faulty Turbo?).
Let’s also guess that the software which reads this pressure signal reacts by limiting the rpm from going any higher when some max pressure value is reached, say 120/130kPa. (Seems like a reasonable reaction),
This scenario would produce the symptom you are seeing.
In the no-load situation, the boost pressure does not get high enough to limit the rpm, and it just tops out at 3200 as usual.
In the load situation the pressure (or pressure perceived by the sensor) reaches a maximum limit and the software prevents rpm from going any higher.
This could also explain the Autoprop effect you saw. The AP may have loaded the engine erratically, such that max boost was reached in an instantaneous way because of the erratic load and the software immediately tried to reduce rpm. The smoother load curve of the fixed prop would probably not affect the engine so dramatically.
Guesswork in the dark.