I just tried putting relictium with fusion only in Forge and the textures don't load correctly.

Relictium is a fork of vintagium which is a fork of Embeddium for 1.12.2
I know that in modern versions of Minecraft fusion works well with Embeddium
Could you patch it in 1.12.2?

I did come across Vintagium before, but they didn't have any public builds. Luckily, Relictium is just available through CurseForge.
It should work now with both Vintagium and Relictium in Fusion version 1.2.8.
Thank you for reporting the issue!