Yes, I started from scratch, even wiped the internal storage. It's the same issue as it was with v5 or v6, can't remember exactly. Dev fixed it then, I hope he fixes it again.
I'm talking about v12 ROM version, not a firmware. I did a dirty flash of v12 over my existing v12 which I installed 3 days ago. Still, heart rate sensor doesn't work, even after dirty flashing it. It worked well with v11.
Try some button combinations like holding the power+up+down or power+up or power+down or power+up+home button. If nothing happens, your motherboard must've gone bad. It happened to me as well, but not by flashing a rom, my device fell.