Powershell – Why does Set-ItemProperty have no effect for IIS applications under Windows 10

Most of our web applications include a Deploy.ps1 Powershell script. Octopus Deploy uses this to configure apps during production deployments, but we also use it to set up developers' local IIS settings. Works absolutely fine on Windows 7, Windows 8.1, and on all our Win2012 production servers.

It's not working on Windows 10, and this appears to be because the Set-ItemProperty cmdlet has no effect. No error message or anything, it just doesn't do anything.

The IIS site api.example.com already exists, and we're using Powershell to create the /myapp application and then change the application's physical path to D:\Projects\Demo

PS C:\> IIS:
PS IIS:\> cd Sites\api.example.com
PS IIS:\Sites\api.example.com> New-WebApplication myapp -site api.example.com -PhysicalPath C:\inetpub\wwwroot

Name             Application pool   Protocols    Physical Path
----             ----------------   ---------    -------------
myapp            DefaultAppPool     http         C:\inetpub\wwwroot

PS IIS:\Sites\api.example.com> set-itemproperty myapp -name PhysicalPath -value D:\Projects\Demo

PS IIS:\Sites\api.example.com> get-itemproperty myapp

Name             Application pool   Protocols    Physical Path
----             ----------------   ---------    -------------
demo             DefaultAppPool     http         C:\inetpub\wwwroot

                                                 ^ THIS IS WRONG!

Running the exact same set of commands on Windows 7, you get identical output but at the last step the PhysicalPath property has changed as expected:

PS IIS:\Sites\api.example.com> get-itemproperty myapp

Name             Application pool   Protocols    Physical Path
----             ----------------   ---------    -------------
demo             DefaultAppPool     http         D:\Projects\Demo

Any idea what's going on? Is there some new admin restriction on IIS10? Some sort of extra elevated permissions I need to modify settings on existing web applications or something?

UPDATE: It seems to work fine if the item is an IIS site, but fails if the item is a web application, which makes me wonder if this might just be a bug in the provider. Any ideas?