Could the object have lifted off the build plate in the corners due to shrinking and warping? If it didn’t (and you’re sure it was stuck down well), is it possible the build plate corners lifted off the magnetic bed?
A troubleshooting step might be to iron some smaller (say 50cm) test objects of similar construction in each of the build plate’s corners to see what the result will be. If you get good ironing, then it could point to a heat related warping problem with the single large object.