GPIOs are set in the build_flags.
‘-DRF_RECEIVER_GPIO=1’ #pin3 on MEGA (uses interrupt number #1)
‘-DRF_EMITTER_GPIO=4’ #pin4 on MEGA
I know of the issues with ESP32/Nodemcu. I’ve ran into them myself too 
Stay away from GPIO0 and 2 if you can. I’m using an Arduino Mega 2560 so AFAIK no issues with “forbidden” pins.
You’ve given me an idea though; Could it be that the message is triggering an interrupt while sending?
I think I’ll attach a debugger and compare OMG vs the Dimmer sketch