I think Mongo does not like for its db folder to be replaced like this in the running Pod.
You can try by turning off Mongo for a moment (scale it down to 0 replicas from the deployment), then create a one-time Pod (non-mongo, you can use an ubuntu image for example) mounting the same volume that Mongo was mounting, and try using this Pod to copy the db folder in the right place. When it's done, delete this Pod and scale back to 1 the Mongo deployment.