opensearch cross-cluster replication stuck in 'BOOTSTRAPPING' state

I’m having trouble setting up multi-region replication between two aws opensearch domains. I’ve been following the documentation (https://docs.aws.amazon.com/opensearch-service/latest/developerguide/replication.html) but once I start the replication via PUT call and query the replication status I see that the status is stuck at ‘BOOTSTRAPPING’ and I can see that no bytes have been transferred, even hours later.

PUT request payload to start the replication:

{
  "leader_alias": "dss-pre-prod-use2-usw2",
  "leader_index": "devportal.resourcev2",
  "use_roles":{
    "leader_cluster_role": "all_access",
    "follower_cluster_role": "all_access"
  }
}

GET response:

{
  "status": "BOOTSTRAPPING",
  "reason": "User initiated",
  "leader_alias": "dss-pre-prod-use2-usw2",
  "leader_index": "devportal.resourcev2",
  "follower_index": "devportal.resourcev2",
  "bootstrap_details": {
    "bytes_total": 0,
    "bytes_recovered": 0,
    "bytes_percent": 0.0,
    "files_total": 0,
    "files_recovered": 0,
    "files_percent": 0.0,
    "start_time": 1650317767399,
    "running_time": 18585600
  }
}

There are only 5 documents in the leader index, so the replication seems stuck. I've tried deleting the follower index and restarting the replication multiple times but with the same result. How can I debug this issue?