This actually happened back in May, but I’ve only gotten the drive to fix it now.
After doing a full system update, I rebooted to the majority of my icons having been replaced by the default unknown icon, as shown in the below images. It didn’t affect all icons, but mostly Steam and Lutris shortcuts, appimages, flatpaks, and programs installed from the AUR.
Checking in the .desktop files for the programs, I can see the icons are as they should be, seemingly with no difference from the ones that do work properly. I’ve also made sure the icons do in fact exist on the system, in (what I THINK is) the location they’re supposed to be. An example for a Steam shortcut:
I’ve tried changing the icon theme, regenerating icon cache, deleting all caches, updates and reboots, and my icons have never come back. If anyone knew anything else I could try, it would be much appreciated.