How are permissions granted in a OneDrive for Business migration?
The default behavior for OneDrive for Business is that all folders and items inherit permissions settings from the folder that is directly above them in the hierarchy. This means folders inherits permissions from the root, and a subfolders inherit permissions from its parent folder.
Unless specifically configured to do so, MigrationWiz does not remove permissions during a migration. If you find that folders or items are shared inappropriately after a migration, you may add the following option to your migration project in order to break permission inheritance, remove existing permission, and finally migrate only explicit permissions:
Note that this option can be added in Advanced Options, in the Support options text field, under the Support section. See How do I add support options to a project or to a single item? for more information.