I think the problem you are facing is
"The policy i use to do the snapmirror is the type mirror-vault."
https://docs.netapp.com/us-en/ontap/snaplock/commit-snapshot-copies-worm-concept.html
https://www.netapp.com/pdf.html?item=/media/6158-tr4526pdf.pdf p.18
The relationship type shall be "vault"