I had a friend with a 2012 Leaf that experienced similar degradation. It was a problem with the early model year and Nissan's choice to use passively cooled batteries.
Yes, the Leaf uses passive battery cooling as opposed to Tesla and others with active cooling. Their first-gen battery chemistry was also worse in hot climates. The newer battery is a different chemistry that has held up better.
The 2011/2012 Leafs had major problems with battery degradation in hot climates so much so that Nissan was forced to offer a free battery replacement. So I got it replaced for free. The newer battery is lasting better, but still not great. I think Nissan screwed up by sticking to passive battery cooling.
For reference I live in southern Florida so we have hotter than average climate.
I own a 2012 Nissan Leaf which has had major battery degradation (first battery went under 70% in only 33k miles, second battery currently around 85% at 45k miles = 78k miles total on both). But on this site, the 2012 Nissan Leaf has less than 2 years of data and shows zero degradation during that time. So it seems like the data is questionable.