Weird.
#1 its a Sport so no special stuff in the seat bottom like seat warmer or leather surface.
#2 its only 4 some years old
#3 you appear to be in Indiana, not some place where oppressive interior heat could break down the foam.
Based on my comments, no, I have not had similar experience. I'm going for defective foam, but good luck getting any kind of coverage or help. I had the cracked tailgate TSB from 06-07. I got it warranty replaced and painted. Less than 3 years later the clear coat was bubbling and flaking, along with every other horizontal surface including the horizontal strips of body right under the door windows. Dealer tells Mazda its normal. Less than 3 years!