I would have taken it back to the dealer if the seat back had come loose just within days of them "Fixing" it. My M seats rubbed and the dealer replaced the backs. No issues other than the driver seat flexed, (loose bolts) the fixed this pretty quickly.
I would offer that maybe a small amount of silicone adhesive might do the trick. Just in a couple key spots along the seat back to help hold it down with the fasteners.
2013 335is Convertible
MODS:M3 Strut Brace / Smart Top Module / Ikon Lip and spoiler
Previous: 2004 3.0 Z / 2008 Z4M