Sure as heck sounds like a defect to me. I have owned dozens of aircraft with plastic canopies, that have all sat in direct sun for many hours for many weekends, and never had any kind of shrinkage or warping problems on any of them. It simply should not be an issue. Sounds like a plastic material formula issue with the original canopy.

Maybe try to find a generic replacement canopy from Great Planes or Sig?

Rick