"Role is not valid" when trying to register a new ECS task definition with cross account assumed roles

Okay this is a tricky one, so I'll try and paint as good of a picture as possible.

Given

• Multiple AWS accounts
• Using github OIDC to assume a role in account A
• The github role in account A then assumes a role in account B that allows it to perform actions in that account

This all works well and haven't had any issues except.

When trying to register a new ECS task definition ex: aws ecs register-task-definition --cli-input-json file://task-definition.json

We're receiving the following from the action Role is not valid

I can perform this action without a hitch from my administrator account using the same task-definition, so I believe the task-definition is correct.

What I've tried

• provided the assumed role in account B with the following trusted entities

     {
            "Effect": "Allow",
            "Principal": {
                "Service": [
                    "ecs.amazonaws.com",
                    "codedeploy.amazonaws.com"
                ]
            },
            "Action": "sts:AssumeRole"
        }

• I've given the role in account B full access to all ECS resources
• I've given the role in account B full admin access - (just to test) None of this worked

Github OIDC role - Account A (111111111111)

// Trust relationships - standard github oidc
{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "GithubOidcAuth",
            "Effect": "Allow",
            "Principal": {
                "Federated": "arn:aws:iam::264460841970:oidc-provider/token.actions.githubusercontent.com"
            },
            "Action": [
                "sts:TagSession",
                "sts:AssumeRoleWithWebIdentity"
            ],
            "Condition": {
                "StringLike": {
                    "token.actions.githubusercontent.com:sub": "repo:OUR_REPO/*:*"
                },
                "ForAllValues:StringEquals": {
                    "token.actions.githubusercontent.com:iss": "http://token.actions.githubusercontent.com",
                    "token.actions.githubusercontent.com:aud": "sts.amazonaws.com"
                }
            }
        }
    ]
}
// permissions
{
    "Statement": [
        {
            "Action": [
                "sts:TagSession",
                "sts:AssumeRole"
            ],
            "Effect": "Allow",
            "Resource": [
                "arn:aws:iam::22222222:role/github-deployer-assumed-XXXXX",
            ]
        }
    ],
    "Version": "2012-10-17"
}

Assumed role is designated account - Account B (2222222222)

// Trust relationships 
{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Principal": {
                "AWS": "arn:aws:iam::111111111111:role/github-oidc-deployer-XXXXX"
            },
            "Action": "sts:AssumeRole"
        }
    ]
}
// Permissions
{
   "Version":"2012-10-17",
   "Statement":[
      {
         "Sid":"RegisterTaskDefinition",
         "Effect":"Allow",
         "Action":[
            "ecs:RegisterTaskDefinition",
            "ecs:UpdateService",
            "ecs:DescribeServices"
         ],
         "Resource":"*"
      },
      {
         "Sid":"PassRolesInTaskDefinition",
         "Effect":"Allow",
         "Action":[
            "iam:PassRole"
         ],
         "Resource": "*"
      }
   ]
}

PS We are not using CodeDeploy