SqlAGListener: Fails Test-TargetResource when Listener already exists

Details of the scenario you tried and the problem that is occurring:

When applying DSC again to a SQL cluster that has been configured correctly with an AG Listener (eg. 192.168.0.1/255.255.255.0), DSC will fail the test as within the Get-TargetResource it looks at 'SubnetMask' which seems to always return 255.255.255.255 and does not compare 'SubnetIPv4Mask' which does hold the correct subnet mask.

Running the following PS on the primary node shows what I mean:
$sqlServerObject = Connect-SQL -SQLServer $env:COMPUTERNAME -SQLInstanceName 'SQL Instance Name'
$availabilityGroupListenerObject = $sqlServerObject.AvailabilityGroups['SQL Instance Name'].AvailabilityGroupListeners['SQL Instance Name']
$availabilityGroupListenerObject.AvailabilityGroupListenerIPAddresses

The DSC configuration that is using the resource (as detailed as possible):

Version of the Operating System, SQL Server and PowerShell the DSC Target Node is running:
Windows 2016
SQL 2016 Enterprise
Powershell 5.1

What module (SqlServer or SQLPS) and which version of the module the DSC Target Node is running:

Version of the DSC module you're using, or 'dev' if you're using current dev branch:
Current version

Start in the SqlAGListener Get-TargetResource logic and inspect the AvailabilityGroupListenerIPAddresses properties shown in the reproduction. Re-run the provided PowerShell query against an existing listener, then verify that Test-TargetResource compares the correct subnet mask and no longer reports the configured listener as different.