Delta 3 unstable RTC (real time clock)

Some of my LP's sometimes loose track of time when not powered externally. When external power is applied the RTC starts ticking again. Usually the RTC will keep ticking when external power is disconnected, but sometimes it does not. Is this a known problem? Is there something I can do to fix/prevent this from happening?

I have used fresh RTC batteries, and verified that the RTC+ pin measures > 3V. The battery seems to be well secured inside its clip.

The serial number of one of the offending boards is CN51G-DF-ENT-AG234400840 S70JR200.