One things for sure… The bolts are DEFINATLEY NOT COMMING OUT!! When I tried putting a seat cover onto a new Miyata such as yours, I could grip the bolts, turn and turn(and the bolt was in fact turning with it), but it never came off. So I assure you that there is probally nothing at all wrong with it!
I agree with Dylan. Chances are good that your handle is starting to rip apart. Take the handle off and check to make sure it’s still in one piece. You may need to franken-fix it.
If I read you right, the answer is that the plastic is simply far inferior than the old style Miyata seats. The older style seats had a far more dense plastic - not as flexy - and held up better to the abuse. It almost seems to be that the new style seat plastic had the density of a plastic grocery bag.