Is that a 3mm deck?..yep, same thing I saw,,the deck is put on with a thick sealing coat, the hole is cut and the underdeck reinforcment goes straight on with minimal sanding making for a dodgy bond, especially with the thin edge grain ply and no fillets like yours. That kind of reinforcement is what the original Chesapeak hatches used with minimal fillet at the end of the rib which resulted in the rib pulling away from the hatch when flexed and the hatch rib 'zippering' during conventional rescues that involved climbing on the aft deck. That kind of hatch reinforcement is actually a fine idea,,it just needs to be bonded properly with the under side of the deck well sanded and a bit of a fillet off the ends, but it would be more secure to simply put in two 4mm x 1" strips one on top of the other providing a sheer to sheer support just like the deck beam off the aft edge of the coaming(I assume that's there).
Assuming the underside of the deck is sealed with epoxy be sure to wipe the area you're gluing with some alcohol/water then sand with 80grit to give some bite to the surface. It's possible the boat was constructed with a blushing epoxy and the underside has a slight waxy film that was never removed before the hatch reinforcement was glued on,,it's also possible the curve of the rib is a poor match exacerbating the above possibilities.