I don't have a clear image of the seat frame, so all I can suggest is that you take the seat out and have a look.
You can recline the seat and tip it backwards onto the rear shelf for inspection, which is a lot safer than lifting it out.
Remove head restraint - Six bolts, the seatbelt shoulder bracket and the seat belt warning light wire, which has a connector.
It could be a loose hinge bolt, but it could also be a crack in the frame so check it out.
Anyone who has had a seat backrest give way doesn't want the feeling of terror twice!!
