I can confirm my suspicions that the HRV data recorded by the Edge 840 is often corrupt. I was able to capture HRV data over ANT+ on the Edge 840 while simultaneously capturing RR interval data over BLE with the Polar Sensor Logger. The source of the data was identical, a Polar H10 heart rate sensor. I previously reported I was suspicious with the HRV data collected on the Edge 840 from a Garmin HRM Dual strap. The use of the Polar H10 allowed the simultaneous data collection. I am currently running 20.19 on the Edge.
The green points match. For most of the other Garmin misses a point or more and then reports the sum ± 1 of the missing points. There is one section where the Garmin just dropped a bunch of points.