You’ve not described the migration procedure you used in detail, which makes me think you have not taken special precautions to ensure that Vault is not running whilst you perform the migration. This is mentioned, but not explained very well, in operator migrate - Command | Vault | HashiCorp Developer.
Running migration with Kubernetes in the picture is complicated. You need to be substantially comfortable with Kubernetes and Helm to improvise temporary changes to the Kubernetes objects, to be in a situation where you have access to the target persistent volume, but not have a running Vault. (If you want to see just how complicated, here is a post from me, with a lot of discussion following on from it: Vault backend migration from Consul to Raft (K8S, Helm, KMS auto-unseal) - #2 by maxb)
For a one-time migration, you might find it easier to just run the migration entirely outside of Kubernetes, on a regular server or workstation. Then start up that migrated Raft Vault, and use the Raft snapshot save/restore functionality to save a snapshot and restore it into a new empty (just initialized) Vault cluster on Kubernetes.