Icon previews not rendering

Ever since I updated to macOS Mojave, I noticed that icons within macOS are not rendering their previews. Quick Look similarly is not working. I realize that some apps simply don't support this feature (Sketch, for example), but icons for Preview, Pages, Keynote, etc. files should all render previews when the feature is enabled (it is enabled--View, Show view options, Show icon preview is checked).

The attached screenshots should demonstrate what I'm talking about. So far I've reindexed Spotlight, left my Mac on overnight in hopes that it would re-index, and re-installed macOS (though admittedly not a fresh install). No luck with any of these troubleshooting steps. I called AppleCare and they seem unable to assist.

This is a really useful feature and you don't recognize how helpful it is until it mysteriously disappears!

I really appreciate any insight you may have here.

Many thanks!

Imgur links