I can tell you that during the last trip to the dealer, a couple fast charges jacked up my battery temp to the point it was pitifully slow in terms of fast charging. But it never cut my power for driving.
I have also taken my 2018 leaf up a road near me up into the mountains. In 9 miles you go from 2400 feet to near 7000 feet. It may have bumped the battery temp by one bar but it was no problem at all. No power issues.
And I live in a very hot area.
I would think the problem is related to fast charging.