Trying to add a user to an environment in AWS Cloud9 I get "Member must satisfy regular expression pattern"

Question

I am trying to give read-write permissions to an environment in Cloud9 to a user.

I am using aws cli for this.

$> aws --version
aws-cli/2.9.23 Python/3.9.11 Darwin/22.5.0 exe/x86_64 prompt/off

I used the following command:

$> aws cloud9 create-environment-membership --environment-id <ENVIRONMENT_ID> --user-arn arn:aws:iam::NNNNNNNNNNNN:user/[email protected] --permissions read-write

Once I execute this on the command line, I get the expected output:

{
    "membership": {
        "permissions": "read-write",
        "userId": "NNNNNN",
        "userArn": "arn:aws:iam::NNNNNNNNNNNN:user/[email protected]",
        "environmentId": "<ENVIRONMENT_ID>"
    }
}

However, when I log into the AWS dashboard on the web, I see the following red error message when I navigate to Cloud9 and click on All account environments:

AWS Cloud9 is unable to process the request. Please see the following details:
1 validation error detected: Value 'all' at 'environmentId' failed to satisfy constraint: Member must satisfy regular expression pattern: ^[a-zA-Z0-9]{8,32}$

I assume the regex complains that I have an @ symbol in the user's ARN. However, that's part of their username.

Any ideas?

Also, why did it seemingly go through when I ran the aws cli command on the command line, but then complains on the web interface?

EDIT: added a screenshot of the error