I have this exact issue and it's driving me nuts.
I've tightened every bolt I can access and still no luck. I'm starting to think it's the seat cover shifting on the seat, but it looks like all the clips are secure.
It's such a horrible feeling to have the sensation that the seat isn't secure. There is a noticeable "click" when the seat shifts due to weight transfer in the vehicle.
Have you found a fix? I took it to the dealer and they claimed they couldn't replicate it.
It's noticeable to my wife and my friend who drove it, the dealer just couldn't figure it out so they denied the issue existed.