I used to be a bike mechanic and still am to some degree. Run a home workshop now, started fixing all my riding buddies bikes and went from there really. I've fitted 100's of headsets over the years.
I'm definitely fitting it correctly, even have a park tool headset press.
I have another king headset on my other bike. It's a king devolution with the older rubber oring design that was apparently problematic and wore down people's steerer tubes. I've never had a single issue with that headset, it's been completely creak free.
The headset I am having the problem with is fitted to a tracer 2 frame. The frame was new to me so this king headset was fresh in. It's an inset version on the top cup as my frame has full 1.5 headtube.
I can feel the top cap moving slightly when applying pressure to the left or right on the bars.
The grip lock is just such a small piece of aluminium it just doesn't sit deep enough in to the bearing.
I've tried different forks, wheel, bars and stem so pretty sure it the top headset.
As for facing, no it hasn't been by me and that has been on my mind in case it's not quite right.
I have never had to face any new premium frame, to be honest it looked faced already (not sure if intense do this or not)
I'm going to try a new headset and see what happens. Having a violent creak is so frustrating.
From searching the tinterweb, it would appear a lot of people have this issue with the grip lock system on king headsets, they simply can't stop it moving no matter how tight it's done up.
People who have had the issue fit a cane creek or hope and problem solved apparently.
I've got all this info from MTBR forum so mainly American users I believe.