When doing an Email Migration, you found that you are missing some or all Notes on target mailbox.
If a previous migration was successful and the items were migrated, on consequent delta migration ODM won't migrate the item.
Archiving and retention policies on target tenant moved the items into Archive Mailbox or event delete the items depending on o the settings.
Verify target's tenant Archiving and/or Retention policies and then look for the missing items in Archive Mailbox or deleted items.
