Same thing happened to me when I moved up from a 445 with 54" deck to the X485 with the 54c deck.
The profile of the mold for the new style decks (not sure why, but figure it was to give a more "robust" look) as it was deeper than the 54" decks on the 425, 445, and 455 tractors (which have NO blowout at all when used with the PowerFlow).
Was invited through my dealer to have an owners meeting with the Horicon engineers in the early days (2003) of the new models, and brought that issue up. They knew about it, but had a patch (bandaid) anti blowout plate to add under the deck. Did very little to correct the problem.
Now that 54C deck sits in the shed against the wall for the last 12 years. I gave up and bought a used 425 and 455 with 54" decks that still work great.. the 425 is dedicated to the MC519 cart for picking up leaves and grass if let grow too long. The 455 is the lawn mowing tractor for the 6-8 acres being kept as lawn.
The X485 works behind the sprayer, the broom, the front blade primarily. No mowing because it does poorly. And the deck was leveled per every set-up procedure.. tires aired up, left right leveled, front to back correct, anti scalp wheels correct. Tried hi-lift blades, and gator blades. Have a drawer full of blades for that 54C. Went back to the old style deck and get perfect results on both the 425 and 455.