That's the drawback of the old Airfix design which doesn't give a center spar to hold the wings. It basically depends on the wing root strength for this but if the fuselage is not strengthened, the weight of the wing will sag and loose the dehydral. Works on a 1/72 kit but not 1/24. Moreover the Airfix plastic is soft and not rigid.
I remember this happening to the Spit ages ago.