Not 100% sure when this started, but I am running the latest firmware (5.7) on my 520+ and am having issues with my eTap disconnecting and reconnecting continually throughout a ride and draining the battery. The only way that I have been able to get it to stop is to disconnect the sensor and leave it off, which completely defeats the purpose of this functionality.
I've seen that this has been an issue in the past, but it seems like it is coming up again in some of the more recent firmware.