Just got a 6 II. Struggling getting it up and running. Ordered AA lithiums as they suggested and still not enough power. Also with external. Next will buy the Sony L series.
SD is pretty clear that AA batteries are not going to cut it and at best are there if a mid switch of external batteries are required. I have had luck with just about any USB C external battery at 10,000 mah or higher, so that should not be a problem and if it is there could be a problem with the deck.
That said, I have grown fond of using the Sony L batteries. They do work well and last a long while.
While recording using external batteries with the MPII, I have the AA's as a backup using the 8 battery tray, and when I record using the Sony L batteries, I use a USB C external battery as a backup, so this way in case of any weird emergency which of course would only happen in a very important recording, I am protected.