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

is "test string config" a string or a JSON encoded string?

Not that it should make a difference (I’ve only ever done JSON blobs never a “true” string)

I don’t see anything else obviously wrong.

And I’m assuming your onChanged isn’t “inside” another function? Which could potentially cause an issue. (not tested myself but worth to check, all mine are top level)
Also check your don’t have more than one call kicking about.