https://github.com/NillerMedDild/Enigmatica2Expert/blob/master/config/cofh/world/16_advanced_rocketry_planets_ore.json#L228

The brackets around minecraft:end_stone should not be there.

Confirmed, those brackets are not right. When i read that right, it prevents the generation of endstone clusters on certain planets. I'm wondering why is that even there? Just for flavour or am i reading this file completely wrong?

Yep just flavour :)