Lapis Lazuli material is called "Lapis" when applied to armor, despite being called "Lapis Lazuli" everywhere else in the game

The trim material is not a reference to the item.
All trim material names:

• Iron

• Copper

• Gold

• Lapis

• Emerald

• Diamond

• Netherite

• Redstone

• Amethyst

• Quartz

Note how it does not say "Iron Ingot", "Copper Ingot", "Gold Ingot", "Netherite Ingot", "Redstone Dust", "Amethyst Shard" or "Nether Quartz". That's because the material and the item are 2 separate things.

Not a bug.

Yeah, but Lapis and Lapis Lazuli are the same thing, it just uses a different name for it suddenly. The material itself is lapis lazuli, just like how the material created when using iron ingots is iron.

(Edit: Re attached the image because I was using Canadian English in the original one)

"Ingot", "Dust" and "Shard" are references to the physical shape/formation of the material, not the substance itself. I don't think "Nether Quartz" counts either, since it's referred to as just "Quartz" in other contexts such as its material block, so Lapis Lazuli is a true outlier here.