That is amazing. A shame that the invade button did not activate for it to be possible to own the gas giant. Oh, one last thing I could think of is maybe that is a moon of really low orbit, so maybe through an error it is now inside of the giant? Gas giants can be rather large, and I don't know if Aurora checks for the central body radius when placing moons (though I have never witnessed anything like this with bigger stars for example).
In that case you would still see the population as a target on the attack screen, but not know whether it was the real giant or just a moon. ..It's a thin chance, but the last explainable frontier.