Your issue sounds similar to what I read here in the article below.
Policy Name : MirrorAndVault
Policy Type : mirror-vault
SnapMirror Label: sm_created
daily
weekly
Schedule :This schedule parameter found in a SnapMirror policy rule is not a transfer schedule and will not trigger a backup. Instead, this schedule controls a new feature known as secondary SnapVault Snapshot copies. This feature creates independent snapshot copies on a SnapVault secondary volume without having to create and maintain a corresponding Snapshot copy on the SnapVault Primary. When a schedule is used in a SnapMirror policy rule assigned to a SnapMirror relationship, that rule will create snapshots on the SnapVault secondary volume when the schedule triggers. These rules will not trigger an update and snapshots with the configured label will not be backed up from the primary volume.
If you want your rule to back up snapshots from the SnapVault primary volume, re-create your rule without the schedule parameter.
Other high-level details and caveats for the Secondary SnapVault Snapshot copy feature:
1)This feature is only available for use with vault and mirror-vault type SnapMirror policies.
2) You must have at least one rule in your SnapMirror policy with no schedule parameter.
3) This feature is only available in ONTAP 9 and later.
https://kb.netapp.com/app/answers/answer_view/a_id/1001334