Dropbox 'Team Folders' are owned by the Dropbox team and can only be managed by a 'Team Admin' this can be created through the 'Team Console'. To be able to migrate these folders they need to be linked to a user being migrated.
Under 'Source Platform Server Settings > Dropbox > Account Details > Team Folders Owner' you will need to enter a source email address of the user who will take ownership of 'Team Folders' in the destination. If this is not populated 'Team Folders' will not be migrated.
Dropbox for Business Team Folders can be identified with the following icon.
Dropbox for Business Team Shared with icon is a group within Dropbox and by default assigned to default 'Team Folder' when signing up for Dropbox for Business. This will be in the format of 'Everyone at NAME OF TEAM'
When specifying a 'Team Folders Owner' this user will become the owner of the files and folders. If at a later date the 'Team Folders Owner' changes to another user and delta migration is performed, this will result in the files and folders residing under two different users. For 'Team Folders' it would be recommended to migrate to a 'Team Site' in Office 365 or a 'Team Drive' in Google.
Team Folder to Team Drive Example:
Here's a screenshot depicting how a Team Folder can be migrated to a Team Drive.