If you lift up the cushion, and look at the bar that the cushion pivots on, it moves up and down.
Due to the superb design skills at Nissan, all that holds the front half
of the seat is two of the teeniest weeniest little screws that you ever
did see.
Naturally, if anyone actually sits on the seat these come undone (Who'd have thought that might ever happen
)
This is why if you lean on the front of the seat it tips up.
Anyway, you can tighten them with a bit of trim wrenching.
Bet this is your problem.
I nearly had the welder to mine, but they tightened up ok, and I don't often have a passenger.