PBI Rest API : Datasets - Update Datasources In Group (PowerBIEntityFrameworkOperationException)

Hi folks,

I'm calling the endpoint described here https://docs.microsoft.com/en-us/rest/api/power-bi/datasets/updatedatasourcesingroup in bid to reconfigure my Analysis Services data source. My Analysis Services instance resides on-premise thus is delivered via a Gateway Cluster and Dataset. 

The HTTP POST Body is roughly the following:

{
"updateDetails": [
{
"datasourceSelector": {
"datasourceId": "Datasource GUID HERE",
"connectionDetails": {
"database": "CURRENT_CUBE_NAME",
"server": "CURRENT_CUBE_SERVER"
},
"datasourceType": "AnalysisServices",
"gatewayId": "Gateway GUID HERE"
},
"connectionDetails": {
"database": "NEW_CUBE_NAME",
"server": "NEW_CUBE_SERVER"
}
}
]
}

Sadly the request constantly throws the below HTTP 400 Bad Request error:

"error":{"code":"PowerBIEntityFrameworkOperationException","pbi.error":{"code":"PowerBIEntityFrameworkOperationException","parameters":{},"details":[]}}}

Interestingly, this was all working up until a few months ago.

Any help would be appreciated.

Many Thanks,

Tim.