All of a sudden, icons/glyphs aren't rendering correctly for me on some desktop apps like slack, vscode, etc. This is a Windows 10 machine, regularly updated (v 21H2, OS Build 19044.1586). Anyone know what could be causing this?

AFAIK, windows exposes native icons via a font (Segoe MDL2 Assets is one of those). Check if the font exists on your system

Good suggestion, but unfortunately that didn't seem to work. FWIW: I installed the following set of Segoe fonts: • https://docs.microsoft.com/en-us/windows/apps/design/style/segoe-ui-symbol-font#how-do-i-get-this-font • https://docs.microsoft.com/en-us/windows/apps/design/style/segoe-fluent-icons-font#how-do-i-get-this-font • https://docs.microsoft.com/en-us/windows/apps/design/downloads/#fonts