What is the cause of "Right Pedal Missing"

Towards the end of yesterdays ride I repeatedly got "Right Pedal Missing" message displayed on my Edge 820. It happened once every few minutes during the last half an hour or so. It was a seven hour ride. I have L and R power on my display and both sides kept reporting power data (3s).

The Edge 820 was starting to run low on battery. The 820 went into battery save mode during the last 10 min or so. This shouldn't have affected anything.

I checked the voltage of the batteries (Duracell Lithium 1/3N) and they were 2.72V L and 2.80V R.

I reinstalled the batteries and during todays two hour ride I had no "Right Pedal Missing" messages. Everything worked as normal.

Any ideas what is going on?