Skipping the diatribe - this is becoming a fairly well known issue to those of us who work with eTap on a regular or semi-regular basis. Only Fox is a replacement rear derail. Service was out of them for a long time but we just started getting replacements in so they are now back in stock.

Go to a dealer of SRAM and have them warranty it.

Theu won’t admit this is a known issue and will act like this is the first they have heard of it. They will probably tel you to try stuff and then may send new batteries, but keep at it and you’ll end up with a new derail and the problem will be solved.