This happened at my site too. I had a development instance I was still working on our future schema for. We went from 4.7.x to 4.8.2 because of the security issue but I didn’t check things right away. We discovered that all of our schema was missing from the DB (postgres, AWS Aurora) and unfortunately this environment didn’t have a long enough backup interval on the DB.
There’s bits and pieces of content still there (some references to files uploaded) but all of our models are gone and have to be rebuilt from scratch.
I wish I had some way to diagnose what happened here but so fare there’s nothing that indicates what went wrong.