It depends on the angle of radiation from all the antennas. When the drone is nose down and moving away or nose down coming towards you, the RF coming from the antennas changes angles with the drone.
Ideally with a vertical antenna, you want it straight so the radiation is transmitted towards the horizon 360°. If you angle it, one side will be 'looking' at the sky and the other towards the ground and that's where the signal will go.
Also, relatively little power is transmitted straight up or down from the antenna. The antenna works that way for both transmission and reception.
As for the OP question, what's your signal strength when this is happening? I think you may be right about the heat, though. Can't really say; need more info. What phone or tablet are you using? Maybe it's lagging?