Photoshop;
The culprit is very likely one of the two black plastic "catchers" that make part of the structure (PN 81203003000) that carrys the weight of the subframe & rider (tensile load).
The seat is held in place with 4 slots: tow at the front of the seat ( at he front of the airfilter) & two where the seat rests on the black "sub frame upper part" very close to the tank. It is very likely one of these middle "catchers". It doesn't go into the hole in the seat. Use a hot air gun to bend it a little outwards. It is the one that lays closer to the tank. It leaves a rubbing mark on the plastic base of the seat on the outside of that particular hole.
The lock at the back locates and locks the seat into place.
there are also two vertical "tabs" that fall to the inside of the sub frame upper part. Sometimes one of these tabs rests on the subframe upper part preventing the seat to "sit" correctly.
bloody h**l, it's not easy to write it down clear...
if needed I will make a photo reportage.