Is TCP loopback connection possible between UWP app und it's FullTrustProcess. And if yes, how?

Question

This is black spot in the documentation.

docs say:

• cannot loopback between win32 and uwp; workaround via CheckNetIsolation
• can looopback in the same package
• the Win32 FullTrustProcess counts as "in the same package"
• can loopback between different uwp/packages, if loopbackAccessRules are configured correctly

I tried a TCP connection between uwp and it's FullTrustProcess Companion App, and it didn't work.

Some quotes from MS employees imply, that the FullTrustProcess is in the same package as the UWP and therefore loopback connection should be possible.
But it doesn't work for me.

For example Rob Caplan [MSFT] says:

The loopbackAccessRules are designed to help with connecting to other app packages, not to your own, where (in general) that's not necessary as loopback is allowed. It looks like your desktop app being full trust threw a curveball at how this works. I haven't tracked this all the way down, but the behavior suggests that since it's not running fully in the package's security context it's not being allowed as part of the package.

Source: https://social.msdn.microsoft.com/Forums/windowsapps/en-US/0cdb98ca-e872-4b61-9506-881acdf5e03b/uwpdesktopbridge-can-quotuap4loopbackaccessrulesquot-be-used-to-allow-tcp-communication?forum=wpdevelop