I used to have a forerunner where HR readings would work as expected, switched recently to an Apple Watch SE 3, updated to 26.1, tried today to setup everything but to no avail.
I have given all required permissions in health app regarding TrainerDay, I have reinstalled the app, rebooted everything, connected/disconnected but nothing, started several workouts on the watch, HR shows 0bpm.
It works for me but after our major upgrades about a moth ago, I had to uninstall the apple watch and reinstall our app and watch, and not to mention I always forget to go to our devices tab to enable it.