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I hit this bug today too, but I think the issue is slightly different (details in kubeflow/notebooks#121). Rather than handle both matchLabels and matchExpressions, I believe the label returned here is probably supposed to be the name of the PodDefault, not the first selector in the list

Just adding some more information here:

The matchExpressions is actually working, but the UI/API does not support it.

API: https://<hostname>/jupyter/api/namespaces/<namespace>/poddefaults

{
    "log": "An error occured in the backend.",
    "status": 500,
    "success": false,
    "user": "<Email@Address>"
}