I have found the heart rate recorded during my weekly gym workout to be woefully low, almost exclusively Zone 1 when I am sweating and panting during the session. Heart rate seems accurate enough during cycling, recording all zones including Z5, consistent with perceived effort.
However if I choose "Cardio" rather than "Strength" for my gym sessions the HR readings are perfect, entirely consistent with perceived effort.
This error is usually attributed to the extra arm movements involved in strength training but in my case I am doing the same gym sessions all that has changed is the Cardio vs Strength settings. This suggests there is something wrong with HR reading specifically in Strength mode.
For now I am recording gym sessions in Cardio mode then changing to Strength by editing in Connect after upload.
Jim J