SqlServiceAccount: Fails to change account for SSRS 2016

Problem:

SqlServiceAccount does not work for SSRS 2016, it produced an internal error.

Description

We have successfully changed the service account for Database Services (DatabaseEngine) using SqlServiceAccount however i get an internal error when i try to set the service account for SSRS 2016.

Steps to produce error

SSRS 2016 EE on Windows Server 2016

$cred = Get-Credential -UserName "testdomain.local\testaccount" -Message "Password please"

Invoke-DSCResource -Module SqlServerDsc -Name SqlServiceAccount -Method Set -Property @{
serverName='TestServer''
InstanceName='TESTSSRS'
ServiceType='ReportingServices'
ServiceAccount=$cred
}

Error Produced

An internal error occurred.
+ CategoryInfo : NotSpecified: (root/Microsoft/...gurationManager:String) [], CimException
+ FullyQualifiedErrorId : HRESULT 0x8007054f
+ PSComputerName : localhost

Exploration

I have debugged the code and see that it errors out at SetServiceAccount. ( I also tried SetServiceAccount in standard powershell which also doesn't work for SSRS, but again works fine for database engine)

It looks like SetServiceAccount is not valid for reporting services and we should use SetWindowsServiceIdentity, a method of MSReportServer_ConfigurationSetting.

Has anyone successfully used SqlServiceAccount for SSRS 2016.

Start with the SqlServiceAccount resource and reproduce the SSRS 2016 case using the shown Invoke-DSCResource command. Compare the failing SetServiceAccount path with the mentioned SetWindowsServiceIdentity method on MSReportServer_ConfigurationSetting. Done means changing the Reporting Services account succeeds without the internal error while the existing DatabaseEngine behavior remains intact.