Lets say a user had (pleas note the past simple time ^^) a folder named “Files” in the mailbox on the top level, you have moved the mailbox to O365 Exchange Online…
Yup, exactly this happens if you move the mailbox with folder named “Files”, you won’t get any error, nothing. The folder will be gone and Exchange will use it for it’s internal use.
You might use MFCMAPI utility to reset the PR_ATTR_HIDDEN property to False, to force Exchange to show the folder again, but be quick! You only got some time before the background agent sees it and hides the folder again ^^
Detailed info about it here: https://www.petri.com/mysterious-files-folder
So remember kids, when moving mailbox to Exchange Online – ensure no mailbox has folder named “Files” in it, or you gonna have a bad time.