Custom staging resource group

Specifying a custom staging resource group as per the example in the documentation does not appear to work and there appears to be no clear way to troubleshoot why.

"stagingResourceGroup": "/subscriptions//resourceGroups/",

The process should create the resource group and use it automatically, if the naming convention does not go against the standard resource group naming convention, but that does not appear to be the case.

Even if you create the resource group separately ahead of time, the process does not like it and creates its own IT_xxx resource group.