On my 2018 I sometimes see one of the blue lights flash for a while, I am told that the traction battery is charging the 12v battery. So why the need to separately charge the 12v battery?
Nissan's charging algorithm for the Leaf has always been a little less than perfect - often a lot less, depending on how the car is driven. Infrequent, mainly short trips seem to contribute to a low accessory battery. You can also drain the accessory battery by leaving the car plugged in for a long time (days, usually) after charging has stopped. If you drive the car a lot, this isn't likely to be a problem, except maybe for some 2013 Leafs. If you drive it once or twice a week, and don't drive it for far or long, then you may want to top off the accessory battery at least once a month, with an external charger/maintainer, or at least check the resting voltage of that little battery. If it becomes an issue, then replacing the OEM 12 volt battery with a higher capacity AGM battery usually solves the problem. Why? Partly because those batteries are more forgiving, and the higher capacity carries the battery over the low-charge periods. But it also seems that higher capacity batteries just charge better in the Leaf, suggesting that it was originally programmed for a different type and capacity battery, and that was never changed when they instead went with cheap starting type batteries.