I defined a new attribute icon for Botania's "Pixie Spawn Chance" attribute on various equipment items, and it shows up just fine.

However, the actual order of icons, i.e. whether the pixie icon shows up before or after the ones defined by Quark, can randomly change every time Minecraft is restarted, presumably because the internal sort order of the resource files is not fixed. It would be nice to have some kind of consistency here.

I'll have to look in to this for 1.21.1, but thats good to know.