Apple Watch Heart Rate Glance Shows Same Number Every Time, Or Series Of Random Numbers, Also On Replacement Watch
If the watches are different, and the phones are different, then the only thing that appears to remain the same is you.

You don't mention, but have you ruled out the possibility of there being anything about you? For instance, I remember reading that tattoos could prevent the heart monitor obtaining a clear reading, and there was some talk about people with very black skin also having difficulties. Example article here http://www.bbc.co.uk/news/technology-32537030

The other possibility is whether there is something with the tightness with which you wear it?