Looks like maybe the left and right recliner mechanisms are out of synch. If you take the trim cover off the seat back you should be able to get a look at the synch rod that spans the frame between the left and right recliner pivot points. If they’re out of synch it should be obvious. Might not be a simple fix, but should help you communicate better to the dealer what’s going on. 2018 GMC Sierra - Elevation Edition