Recently when a user tried to look at his Online Archive in Webmail the folder was missing. In Outlook 2016 ( NOT Outlook 2013 as this doesn’t support archive very well ) the folders were listed.

The issue is due to the folder count in Online Archives. The folder limit 10,000 and the current folder count was 15,000

Kindly refer to the below article for more information.

https://support.microsoft.com/en-us/help/2849181/some-folders-are-not-visible-in-outlook-on-the-web