There have been a couple requests to see the file so that we can look at it. When we cannot give a solution it usually means this is not a common problem.
Unless it can be duplicated and repeated then it is usually a physical or software problem at your end.
Without seeing the file people can only make educated guesses. If you don't want to let the people that offered to diagnose the problem help then you might want to try and isolate it at your end.
I'd start by creating a NEW file and just create a much smaller and simpler set of objects, group them in the same kind of way, soften and harden edges, save the file exit and reopen and see if the problem can be duplicated.
This will at least let you know if it's something specific to that file or all files.
"If I have seen further it is by standing on the shoulders of giants." Sir Isaac Newton, 1675