Battery to derailleur paring is random, as I charge batteries and move them around randomly w/ no real pattern. Example, heading out for a ride, taking bike #1, but not sure when charged last. I'll grab batteries from bike 2, and put these on charge. or maybe bike 2 has no batteries, so I'll take the two off charge and put on bike, and put these on charge. But charger to bike is random. And bike to bike, I'll intentionally swap front to back on the assumption that the back always gets more use.

Problem has stayed w/ Rear Der on Bike #1, and has not appeared anywhere else.