I have a garmin instinct solar, I’ve noticed a few times now while cycling in zone 2, I begin to roll downhill fast and my heart rate sensor is displaying 180 plus when in fact it’s between 90-110. This must be a fault.

You probably experience a known limitation of wrist-worn optical heart rate sensors, which struggle to operate properly when your wrist vibrates when going fast on a bike.

Sounds feasible. Was thinking it has to be something like that