Email folders disappeared from navigation but still exist on IMAP server

More than 20 email folders suddenly disappeared from my Xfinity webmail folder navigation. The folders and emails have NOT been deleted. Xfinity still sees the missing folders in the search folder selector, and when I select a missing folder and search it, Xfinity successfully returns the emails stored in that folder.

Outlook's IMAP Folder Query also sees the missing folders and reports them as subscribed. However, the folders don't appear in the normal folder tree in either Xfinity webmail or Outlook. This appears to be a server-side folder hierarchy/index problem.