[BUG] Cross account iam roles do not seem to work

[pastibog]

Describe the bug
Trying to send data from a EC2 instance to a OS cluster in a different account with IAM roles don't seem to work.

To Reproduce

• created policy on logstash EC2 (Account A) that can assume a role (in OpenSearch account - Account b) that is "godmode" over the cluster
  Account A policy attached to logstash EC2:

{
    "Statement": [
        {
            "Action": [
                "sts:AssumeRole"
            ],
            "Effect": "Allow",
            "Resource": [
                "arn:aws:iam::*:role/<opensearch_role>"
            ]
        }
    ],
    "Version": "2012-10-17"
}

Account B opensearch_role:

{
    "Statement": [
        {
            "Action": "es:*",
            "Effect": "Allow",
            "Resource": [
                "domain_arn"
            ]
        }
    ],
    "Version": "2012-10-17"
}

Account B opensearch role trust relationship:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "",
            "Effect": "Allow",
            "Principal": {
                "AWS": "arn:aws:iam::<account_a>:root"
            },
            "Action": "sts:AssumeRole"
        }
    ]
}

• updated the upper mentioned role’s trust relationship to allow logstash EC2's role to assume it
• tested that the EC2 instance can assume it with a aws sts assume role, works fine

Redacted logstash config:

  opensearch {
    hosts => ["https://host:443"]
    index  => "test-name-%{+YYYY.MM.dd}"
    ecs_compatibility => disabled
    auth_type => {
      type => "aws_iam"
      region => "eu-west-1"
      service_name => "es"
      profile => "research"
    }
  }
}

We do something similar with fluent-bit where that works fine.
Was wondering if this is not supported right now.

[pastibog]

Forgot to mention that our config file looks like this:

[default]
region = <region>
credential_source = Ec2InstanceMetadata

[profile research]
source_profile = default
region = <region>
role_arn = arn:aws:iam::XXXXXXXXXXXX:role/role_in_account_B

[dblock]

[Catch All Triage - 1, 2, 3, 4, 5]