on Fenix 7S (possibly on others, too) i observed that using an external HR Sensor will cause wrong values by use this way:
I mounted the ANT+ Sensor aroung me ( new TickR used with ANT+ full battery and it is showing that it detects HR)
in the past the watch did notice me, that a new Sensor is connected but in 8.35 and 8.37 not. ...
Than i went outside, did start a RUN activity, with a prepaird training... and PLAY....
The measured HR rate did not increase much only some beats (and this is not why i'm good trained or slow).
After around 1km i PAUSED running because i feel my heat beating fast, but Watch is showing up only 80BPM and decreasing.
In Menu of SENSORs: the Sensor is mentioned as CONNECTED, but the values are not used for running. That are "Zombie" values.
I can disconnect and reconnect and it doesn't matter...
IF i deactivate now external ANT+ Sensor, the internal wrist Sensor becomes active and with in 10s the HR goes up from 80 to 140bpm.
I'm quite sure, In the past by using a run activitiy the watch notifies my using an external sensor. with 8.3x not. Mentioned again, Sensor is okay.