Unable to remove session host "Session host not found"

I'm trying to delete a test host pool that I created, but I'm unable to remove the session hosts. I have deleted the RemoteApps and the AppGroups, but when I try and delete the session host I get a powershell error:

WARNING: One or more errors occurred.
Remove-RdsSessionHost :
ActivityId:
Powershell commands to diagnose the failure:
Get-RdsDiagnosticActivities -ActivityId
At line:1 char:1
+ Remove-RdsSessionHost -TenantName $TenantName -HostPoolName $HostPool ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : FromStdErr: (Microsoft.RDInf...eRdsSessionHost:RemoveRdsSessionHost) [Remove-RdsSessionHost], RdsPowerShellException
+ FullyQualifiedErrorId : DefaultNoRdsError,Microsoft.RDInfra.RDPowershell.SessionHost.RemoveRdsSessionHost

I cant get any detail from the Get-RdsDiagnosticActivities command, as the activity doesn't appear for the command in the list.

The session host is there, I can see it using get-rdssession host, and I'm using the FQDN to try and delete, which exactly matches the SessionHostName that is outputted via the get command. If I use the shortname instead I get a different error saying "Specified SessionHostName %servname% does not exist in HostPoolName %hostpoolname%" (and that error is outputted into the RdsDiagnosticActivities)

Anyone run into this? I've tested creating a new host pool / session hosts, and the same is evident, I cannot remove the session hosts.

Thanks

Ben