So I've flown a few more flights to try and figure out what is happening.
There is no slow battery drain. Rather the main battery and/or the APU battery go from full charge 98-100% down to 0% instantly. Causing the respective battery offline message to appear in the EICAS. Study page confirms 0V, as does the DC elec page which also has the battery no longer green. It seems to be able to occur in any stage of flight: on ground, in cruise, on approach. Reloading to before such an event took place results in it not happening. Whenever it does happen, the HYD page shows a display overflow of digits. The battery does charge again subsequently. There are no active failure scenarios.
Below are some screenshots of a new airframe (newly created and first flight) on its maiden flight with the main battery dropping to zero after being enroute for around 45-50 minutes. Showing DC, AC, HYD, overhead panel, electrical study page for the failed battery. The summary was taken a few minutes later.
I'm running the latest stable xplane 12 version on an M1 mac.