I have WIN 10, Outlook 2013, and I just discovered this also. LAME !!!
IMAP Comcast connection
Here is a solution:
With Inbox selected, right click top item for the account (Mine is Comcast IMAP)
Then select IMAP Folders
Then query for a lost folder
Click the found folder and then Subscribe
UN-CHECK "when displaying heirarchy in Outlook, show only subscribed folders"
BINGO - they are back, will take time to re-sync them
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