I agree with Murphy
The problem occurs when the received signal is coming through the prop and the effect is the same on strong and weak signals. This rules out many possibilities.
What may be happening is that the incoming signal is modulated by the prop, quite literally mechanically chopped. This will put strong amplitude modulation onto the carrier. With a two blade prop running at 2400rpm this is 80Hz am. The modulation will be a constant proportion of carrier and so bear a constant relationship to the received signal (also am). Presumably the receiver agc sees the chop and turns the gain down, reducing the amount of wanted signal.
The effect may be specific to a particular type of receiver or even to the mode of operation set on the receiver or the software version loaded. I don't have a schematic so can't even begin to guess what is going on inside the receiver.