It seems to me that your car seat's base is too wide. The buckles on the rear seat bottom cushion are positioned so that they are held in place to ease lowering the rear seat bottom cushion if you fold it up as part of making a flat load floor. The buckles on cars that don't have folding seat bottoms are ususally on loose belts and not on a storage bracket that facilitates the folding seat.

This is an unfortunate situation but I don't think Volvo has a solution nor do many of the other car makers that make wagons with folding rear seat bottoms that have made folding their seats easier.