It's the nature of the beast. With a full mulch kit, the air flow has been compartmentalized in each chamber and it has to go somewhere, so down and out around the front is where it goes due to the direction of rotation of the blades and the lift/sail on said blades. Sometimes you can buy a separate part called and anti-blowout plate that will bolt to the bottom edge of a deck, but I doubt Hustler has one for that particular machine and deck. Maybe you can make one. It isn't hard to do really. I have made a few out of heavy gage aluminum plate and cut and bent it to the shape I needed and bolted them on.

This is a factory made anti-blowout kit for a JD garden tractor with a 54" deck. It bolts to the underside of the lip that runs around the left side and front of that deck-

 

Guy posted this the other day. Said its a piece of plastic landscape edging.