How can I PATCH omaSettingStringXml?

Question

calling

PATCH https://graph.microsoft.com/v1.0/deviceManagement/deviceConfigurations/<omitted>

with the following payload

{
    "@odata.type": "#microsoft.graph.windows10CustomConfiguration",
    "omaSettings": [
        {
            "@odata.type": "#microsoft.graph.omaSettingStringXml",
            "displayName": "Approved USBs Group",
            "description": null,
            "omaUri": "<omitted>",
            "fileName": "<omitted>",
            "value": "PEdyb3VwIElkPSJ7NjVmYTY0OWEtYTExMS00OTEyLTkyOTQtZmI2MzM3YTI1MDM4fSI+CiAgPE1hdGNoVHlwZT5NYXRjaEFueTwvTWF0Y2hUeXBlPgogIDxEZXNjcmlwdG9ySWRMaXN0PgogIDwvRGVzY3JpcHRvcklkTGlzdD4KPC9Hcm91cD4="
        }
    ]
}

results in 503 DQCancelledOnRequestTimeout. Am I doing something wrong?