All of the permissions at the Source set on a Public Folder have to be supported by the Destination in order for them to be included in the migration. The most common permissions that cannot be migrated are public distribution lists. Public Folders in Exchange Online do not support public distribution lists (email mailboxes set as MailboxType.PublicGroup) as valid permissions. Because of this, we have to strip these types of permissions out of the set of folder permissions when migrating.
Note: Individual user permissions will still be migrated, but public distribution lists will not.