How do I not receive "Internal Failure for IAM authorizer" error when using AWS IAM authorizer on Govcloud?

I have an app which uses a role with this policy to invoke an API gateway:

    {
        "Version": "2012-10-17",
        "Statement": [
            {
                "Effect": "Allow",
                "Action": [
                    "mobileanalytics:PutEvents",
                    "cognito-sync:*",
                    "cognito-identity:*"
                ],
                "Resource": [
                    "*"
                ]
            },
            {
                "Effect": "Allow",
                "Action": [
                    "execute-api:Invoke"
                ],
                "Resource": [
                    "arn:aws:execute-api:us-east-1:XXXXXXXXXX:aaaaaaaaaa/$default/POST/routename/${aws:PrincipalTag/username}"
                ]
            }
        ]
    }

(In govcloud, us-east-1 is changed to us-gov-west-1).

This works fine in commercial. However, I get 500 internal server errors on govcloud. Upon customizing and inspecting the logs, I find that it's an authorizer error with the error message "internal failure for IAM authorizer".

Searching this error on google yielded 0 results... Now I'm scared.

In a panic, I tried opening up all permissions more broadly

    {
        "Version": "2012-10-17",
        "Statement": [
            {
                "Effect": "Allow",
                "Action": [
                    "mobileanalytics:PutEvents",
                    "cognito-sync:*",
                    "cognito-identity:*"
                ],
                "Resource": [
                    "*"
                ]
            },
            {
                "Effect": "Allow",
                "Action": [
                    "execute-api:*"
                ],
                "Resource": [
                    "*"
                ]
            }
        ]
    }

But this yielded the same results.

However, when I tried hitting the same endpoint using complete admin permissions, my requests went through just fine.

What can I do to stop this behavior? Are IAM Authorizers even supported on govcloud? Do I need to add more permissions?