I didn’t bother posting a wide shot because you can’t tell what it is from so far away. It just looks like it’s hazy from dirt. But actually I was able to pick some small hunks off with tweezers and it has an appearance and consistency similar to dried glue. It’s slightly white and semitranslucent. It is also only present on the interior surface of the screen.
But as requested, here is a wider shot comparison of the normal unaffected screens (top) with the affected, coated screens (bottom). imgur.com/vurLJvw
My running theory is that something was glued to the screen at some point, perhaps to deflect sunlight. Or the screen was coated in something, maybe to try to further prevent the intrusion of bugs.