I too was just using the radio, no headset. I think this makes sense. I would assume transmitting requires substantially more power than receiving. It would make sense that when you push the PTT button the voltage drops enough to where the radio shuts off.
Shameless plug for my airsoft chrono calculator, useful for converting Joules to FPS/MPS and determining what FPS difference there would be assuming no loss or gain to Joule creep.