I have one user this has happened to 3 times, and cannot figure out why it is happening. Maybe we can find a correlation. I do not think it is server environment or SM 15 vs 16 related., but a bug across all versions. The first thing I saw in my users account is they have about 20 sub-folders under the Inbox. They also IMAP with Outlook 2016, and an Android Turbo. I see your user is using Outlook 2017, how about a large amount of sub-folders?