Folder tree hierarchy changed on its own

In v5.0.17263, I have two email accounts – call them “account1@gmail.com” and “account2@gmail.com”. Until today, both of them had the identical folder tree:
account1(or 2)@gmail.com
Inbox
Outbox
Sent
Trash
Drafts

Suddenly, the tree for one of the accounts has changed, as follows:
account2@gmail.com
Inbox
[Gmail]
Outbox
Sent
Trash
Drafts

The contents haven’t changed – there’s just the sudden appearance of “[Gmail]” in the tree.

Anyone know what causes this? And is there a way to get things back the way they were (and that account1 still is)?

Thanks!

Have you created a folder recently? It may cause this because it is named like: Gmail/Folder