Okay. Who HASN'T had a 12V battery issue with their 2023-24 Prime?

Agreed. My bolt recommends leaving it plugged in when not in use.
It keeps both batteries conditioned that way

 

They use a connected battery.
https://static.nhtsa.gov/odi/tsbs/2020/MC-10178779-9999.pdf

 

What makes a long trip, and how frequent, for keeping the 12V charged?

The annual car mileage in Japan is under 7000km, or well under 5000 miles. If this was mostly an issue of the owners not taking proper care of the 12V, it would seem there would be a lot of dead ones there.

There seems to be a case where the software isn't putting a system into sleep mode under certain conditions. Though I think that bad batch of batteries played a part. Whether they were bad from a defect or improper storage doesn't matter.

Owner's manuals provide two things; important information for the operator, and a CYA for the manufacturer. I believe the warning Toyota gave about the 12V battery and charging is the latter.

The 12V is charged while the traction pack is being charged, the battery heater is own, and whenever else the car calls for power from the outlet. The addition drain happens when power isn't called for, as the system monitoring for the conditions to call for power. Keeping the charge cord depowered when not needed is a safety feature.

Other EV makers have 'maintain the 12V' as a reason for the car to draw power from the outlet. Some even will do so from their traction pack when not plugged in. Toyota likely also do so, but the drain over a couple days likely isn't enough to be a concern. That is until other issues with the 12V are present.

You got a like for keeping it polite.

 

This is why the traction battery charger should have circuitry to know the status of the 12V battery and include a charger. The charge cord should remain powered until both batteries are satisfied. While the charge cord is plugged in, this charge circuit should check both batteries at least once each day to see if they need to be topped off. The 12V can power this and would only need a few micro-amps if done properly.

Option 2 is the traction battery always keeps the 12V battery maintained as long as the car is plugged in whether it's charging and the cord is energized or not. If the car is unused and still plugged in, the traction charger may need to pump the main battery up when it drops below 95%, but it's not going to be very often.

This isn't a challenging nut to crack.