The bug:

When attempting to use a resource pack that changes the game's title bar/taskbar icon, the icon files are simply not loaded. No message was printed into the console log related to this and the files are the correct 16x16 and 32x32 sizes. I am unsure if the issue occurs on MacOS as I'm unable to test it however the pack does include properly formatted minecraft.icns files for MacOS. No other resource packs have conflicting files either. Unless the pack is not properly formed, I see no reason why this should be happening as the icon is a regular Minecraft asset that the launcher downloads.

If someone could look through the pack to see if it's malformed that would be nice, in case I made a mistake I couldn't spot.

How to recreate:

Download the attached resource pack. When loaded, the icons at assets/icons and assets/minecraft/icons are ignored and even after a restart the game retains the crafting table icon downloaded by the launcher.