I found the same thing sailing the SX in 20+ downwind without the chute. We were way in the lead on one race when we caught a big puff and began to stuff. I jerked the steering to head dead down but she just dug in and slowly flipped over forwards - turtling with me under the tramp!

We learnt to uncleat the jib at the first sign of digging in after this incident. This saved us on a number of occasions. It seems the jib really drives the bows down.

We never pitchpoled with the chute up - mostly capsizes whilst learning to sail with one hull out the water or getting over ambitious carrying it on a reach in light air.

Chris.