I've never heard of folding the bag causing cracks, nor has it ever happened to me. Unless if the bag was stored incorrectly or in a bad atmosphere to cause something so drastic.
i had the same exact problem! i bought a speedy MC 30 last month and the dustbag was too small..i couldn't go back to ask for a new one because i was travelling out of the country that i bought the bag in...i was really disappointed because this was my first LV bag and all my gucci's came with a nice big dustbag
Canvas speedies always come with the small dustbag but with other line of speedies come with bigger one.