When I'm in the HR broadcast screen and hold the BACK button, it prompts me to stop broadcasting.It should show the watch face as written in the instructions.
Is it possible that the hotkey 'hold back' was redefined to do something else? In any case you may be able to restore the hotkey to go back to the watchface, in the System settings.
Disclaimer: I no longer have a Fenix 5 Plus. Newer software may be different than my experience.
Unfortunately it doesn't solve it, but thank you because I had never seen this configuration and I set that it must start the FC transmission when I press and hold the start button.