Same for me.
I can make a get request to: https://graph.microsoft.com/v1.0/users/SOMEUSERID/mailFolders/
But when I try to get: https://graph.microsoft.com/v1.0/users/SOMEUSERID/mailFolders/PublicFoldersRoot/childFolders
I get an error result for any user of this domain:
"message": "Mailbox move in progress. Try again later., The user and the mailbox are in different Active Directory sites."