Iron Chests chests do not count as chests for the purposes of the I Need More Data achievement. Discovered because I still have one vanilla chest in my house :-)

Well, I figured out why this is occurring: Iron Chest blocks aren't part of the common chests tag. In fact, as far as I can tell, they're not part of any block tag that I could make use of for this. I suppose I could create a custom tag that includes both them and regular chests, but that feels a little special case-y to me. I think I'm going to let this one go for now. Thank you for the report, though!