Out of curiosity, if you take the FD battery and put it in the RD does the issue persist?

I had a similar issue (only it was the FD) and swapping the batteries resolved the issue. Interestingly enough, if I switched the batteries back it would crop up. It was intermittent though, like 1 out of every 50 shifts.

Ultimately I convinced SRAM to swap the battery and they did so via an authorized retailer (where I also bought the group from) with zero hesitation and it went away.