Configuration is always empty when using window.Twitch.ext.configuration

No

That could suggest you didn’t set the segment you think you did.

Or you might have the config service disabled on this version of the extension (version being an extension version, not a config version) and you wrote to a different version (if you have multiple versions).

So check the extension has the config service enabled in the console.
Reinstall the extension to the test channel.
And test it again

If you are testing in the rig, you might have set a different channel for the view than you get setting/getting against using postman. So check that too.