I wanted to share this fix because I know I'm not the only person who's run into this issue, and it's not the kind of problem you'd expect:
If you're having trouble with saves, such as quicksave not working until you press escape, screenshots on your save files being black or not matching the save location, Enable TAA (Temporal Anti Aliasing) in your game settings or SkyrimPrefs.ini.
I know it sounds crazy, and it is a stupid bug to have in the first place, but this worked for me. Another user said that their saving issues went away when they disabled FXAA.
My problematic setup: both FXAA and TAA were disabled in-game as I was using MSAA via my Nvidia settings. Saving the game was not working properly.
Working setup: in-game FXAA disabled, in-game TAA enabled, MSAA still enabled in my Nvidia settings. Saving the game now works properly.
It's a bit of a bummer since TAA does make the game look blurry imo, but it's better than having a busted save system.
Bethesda, PLEASE notice this and make a real fix; it shouldn't be a problem to play the game without in-game antialiasing.