Eventbridge bus: can't receive messages on custom event bus?

Your custom bus will not receive any "aws.ssm" events. All aws.* are going to default bus only. The custom bus can only receive custom events from your application, e.g.:

    "source": [
      "myapp.test"
    ]

From docs:

When an AWS service in your account emits an event, it goes to your account’s default event bus.