Just thought to come back and post an update.
Nissan eventually replaced the Entertainment Unit under warranty and the issue is now fixed. It successfully updates with new software and maps as they become available.
I have the same issue:
https://www.reddit.com/r/leaf/comments/w7t12o/help_with_updating_the_entertainment_systemsatnav/
I've been going round and round in circles with Nissan Connect support for 2 and a half months and I'm still nowhere.
