Lambda Data Action 403 Forbidden despite correct IAM policy

Here is the Data Action config I’m using in the Architect flow:

{
 "name": "CallLambda",
 "type": "invokeLambda",
 "configuration": {
 "arn": "arn:aws:lambda:eu-west-1:123456789012:function:GenesysCallback",
 "payload": {
 "contactId": "{{contact.contactId}}"
 }
 }
}

The flow executes but returns a 403 Forbidden error. The IAM role attached to the Lambda function has lambda:InvokeFunction permissions. I’ve checked the trust policy and it allows lambda.amazonaws.com to assume the role.

Genesys Cloud documentation says the platform uses a specific execution role to invoke the function. I’ve added the Genesys Cloud account ID to the resource-based policy on the Lambda function as well:

{
 "Effect": "Allow",
 "Principal": {
 "AWS": "arn:aws:iam::987654321098:root"
 },
 "Action": "lambda:InvokeFunction",
 "Resource": "arn:aws:lambda:eu-west-1:123456789012:function:GenesysCallback"
}

Still getting 403. Is there a specific region mismatch issue? The Lambda is in eu-west-1 and the Genesys org is also EU. Maybe the account ID in the principal is wrong. I’m not sure where to find the correct Genesys Cloud execution role ID.