How do I know my Rhythm 24 is being used during an activity

I have the Fenix 6x and the Rhythm 24+. I noticed during my last activity that the HR gauge on the data screen seems really of like it was using the wrist based reading and not the external HR monitor. The LED on the Rhythm was showing zone 4 while the watch was showing zone 2. So I thought if I used a power mode that disables the wrist based reading during activities it would pull the data in from the external monitor, and I would KNOW that I'm getting the reading from the Rhythm. BUT... of course when I did that the watch showed no HR reading at all. So.. Any thoughts on this? Why does the power mode idea not work?? How can I know I'm getting the Rhythm's reading ??