loading SMAppService LoginWindow agent

I have an app that I would like to register a launchd agent at the login window.

my plist looks like this:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>RunAtLoad</key>
	<true/>
	<key>LimitLoadToSessionType</key>
	<array>
		<string>LoginWindow</string>
	</array>
	<key>AssociatedBundleIdentifiers</key>
	<string>com.myapp</string>
	<key>Label</key>
	<string>com.myapp.login-window</string>
	<key>ProgramArguments</key>
	<array>
		<string>/usr/local/myapp.app/Contents/MacOS/myapp-agent</string>
		<string>--login-window-agent</string>
	</array>
</dict>
</plist>

If I manually copy the com.myapp.loginwindow.plist to /Library/LaunchAgents and load it manually it works as I'd expect

If I use SMAppService though it does not work:

let service = SMAppService.agent(plistName: "com.myapp.login-window.plist")

do {
    try service.register()
} catch let error {
    print(error.localizedDescription)
}  

I get:

The operation couldn’t be completed. Service cannot load in requested session.

That makes sense as I'm not at the login window when I run that routine.

However checking service.status() (with formatting) I get

LaunchAgent(com.myapp.login-window.plist) status: Enabled

which would suggest that it's loaded, yet when I log out or reboot and I'm at the login window the agent is not running. Indeed the agent does not appear in launchctl list

I'm not sure what else to check to see what's going on.

I've tested loading LaunchAgents and LaunchDaemons in other contexts using the ServiceManagement framework and these all work as I'd expect once approved. It's just the LoginWindow agent that is having issues.