Iron veins are sometimes missing iron in some sections

I found a large vein of tuff that didn't generate in the usual blob-like way but like how large iron veins do. I mined a large amount of it without finding any iron at all. I found a normal large iron vein without having this issue elsewhere in the same world.

Seed: -6294623093073275199

XYZ: 150 / -50 / -340