Smelting alloys and such with the end stone smelter currently does not grant any experience for taking the items.

Are you sure you don't mean the End Stone Furnace, which does indeed give experience? I have just re-tested the End Stone Smelter with the ancient debris to netherite scraps recipe and can confirm that no experience is given.

As a side note, when breaking the end stone furnace, I noticed the invalid material texture as the breaking particle effects (OptiFine if that affects it).

Hmm, you are right, there is definitely something strange going on here. Sometimes I get exp from the end stone smelter, sometimes not. I'll have to investigate further.

Could you clarify exactly what you mean? From my testing the end stone smelter does give exp, just like the vanilla furnace.