(rainbow etching applied)
forge 1.20.1-47.1.3
quark 1.20.1-4.0-439
aether 1.20.1-1.3.1-neoforge

I think it might actually have to do with curios generally, I tried to apply it to an elytra and equip it to the back curio slot with elytra slot and it also did not have the runic etching.

The rune colors have to be applied individually to the renders. This isn't fixable without specific interaction code for each mod that uses a glint effect.