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Is that a topic rule, not a namespace one?

Hmm, I see you created the rules yourself:

const sendEventSourceKey = new eventhub.EventHubAuthorizationRule("send", {
    resourceGroupName: resourceGroupName,
    namespaceName: eventHubNamespace.name,
    eventHubName: eventHub.name,
    authorizationRuleName: "send",
    rights: ["send"],
}, { parent: eventHub });

The one I'm trying to access is the default namespace SAS Policy. Here what I see from the ARM template:

{
    "type": "Microsoft.ServiceBus/namespaces/AuthorizationRules",
    "apiVersion": "2017-04-01",
    "name": "[concat(parameters('namespaces_mynamespace_name'), '/RootManageSharedAccessKey')]",
    "location": "Australia East",
    "dependsOn": [
        "[resourceId('Microsoft.ServiceBus/namespaces', parameters('namespaces_mynamespace_name'))]"
    ],
    "properties": {
        "rights": [
            "Listen",
            "Manage",
            "Send"
        ]
    }
},

😁