I would get LeafSpy and have someone examine the readings when the car is not regenerating. It is probably not safe to do so while driving so get an assistant to ride with you.
One possible scenario is that the battery temperature sensors have intermittent faults and this is causing the BCM to reduce or disable regen. I think the regen is limited when the battery is cold so a faulty temperature reading could possibly cause regen issues. This assumes the faulty temp sensors don't set some sort of trouble code (or your dealer missed it...).
Of course, that is just a wild guess, so IMO the best approach would be to get as much real data as you can while the fault is occurring. The only way I know to do that is with Leaf Spy running while the regen is not working.