Unfortunately you are probably correct, especially in cold locations and if the car isn't driven more than a couple of times a week most of the time.
Did you by chance test the voltage from cig lighter for pwr on and pwr off. That will be the first thing I do if I get the warning... from any place about charge issue. 900 miles seems crazy. Almost like an overhead light was left on...
We had a Volvo XC60 and every 3-5 days of non-driving -- dead 12v battery. Why? Apparently GPS signal seeking draining the battery... solution, disable that module or add a hideous battery tender (latter was done in the garage for a 7-day trip). MME has a lot of tech that stays alive during off position. Clearly so long as HVB has a charge, the 12V should be kept tendered... why not.. Its like standby on a cellphone vs on the phone.
That build date and error would fall in line with bad LG HVB cells?
I've owned 3 hybrids (2 different Prius and a Highlander) and an EV (Bolt EV). The 12V batteries in all but one lasted more than 5 years. The Highlander's battery dieing was likely due to the fact that the car sat in the driveway not being driven for a month or two at a time, but I would constantly walk past it with the smart-key for it in my bag. Which made the car's interior lights come on. 5 weeks of that eventually drained the battery. During the early part of Covid, that car wasn't being driven much at all and I had to remember to start it every month just to keep the battery happy, or periodically put it on the battery tender for a day or two.
I mean it wouldn’t surprise me, but it’s just strange all the local driving that is done, around 140 each day and using the battery consistently and to have this type of error. The 12v is dead once this error happens and it has to be jumped to even open the doors. Same identical thing to what happened in August.
Maybe both have an issue and so the HVB is dragging the 12v down. I do hope you get resolve. I have 190 miles (since 12/8)...
My partner drives 140m total each day (or more). It now has 27k on it, had it since Feb. Would have expected more issues if it was the high voltage battery like charging or range but neither are an issue at this time. ??
?? 
What happens when people go on holiday and leave the car at the airport for 2 weeks? Surely the 12v system should be robust enough to cope with this. Bjorn Nyland left his Tesla Model 3 (Sorry)
for a while, could have been 2 weeks, definitely over a week, in freezing temperatures at the airport and there was no problem in starting. What happened to all the prototype testing etc? Or did they assume the cars would only be sold in California?
Have no idea but do you think the 12V would have been tested back then and determined good? And dead this fall. When started I saw the new one charge itself. Why is a brand new battery sitting at 75% is a question. However if it charges when started up and it wakes up to charge the HVB, depart, remote etc. it should be able to be programed to wake up and keep the 12V at 90%? Something is a bit confusing with mine? Looking for the warm fuzzy feeling about running it again I guess. Only monitored it once and no conclusions but I do wish it had of been sitting at a bit higher SOC. Possibly no prob but strange?
I would think they will not sell one without a block heater in the northern part of Wisconsin . Just a big resistor that replaces a plug in the block. Heats the water jacket and less wear at start up crank. Was just mentioning that you can plug it in for maintenance when it gets cold and it will start every time. The cab heats up a lot quicker when the fluid is a bit preheated. Cheaper and greener than idling it for 30 minutes so your bum is warm. A lot of office parking have them here. Would be a good green incentive for your office up north.
The MME battery is an AGM. The “deep cycle” nomenclature applies only to old lead-acid batteries. My buddies with bass boats have all replaced their trolling motor deep-cycle batteries with AGM because they perform better and will last longer.
Lithium batteries have issues also. You can't charge them below about 32* without pre-warming them. They also suffer from the loss of capacity as they get cold just like the HVB.
