When anvils were introduced, they weren't put into any sound category (stone, dirt, iron, glass etc). Instead, they had specific sounds for placing, using and breaking, that were added exclusively for them. This meant that the anvil events that didn't get a sound never got a sound event at all, what caused MC-11996. In the recent snapshots, this bug was fixed, but the anvils got stone sounds instead of iron. This is the exact same thing that happened with hoppers - just with wood sounds instead of stone -, as described (and fixed) in MC-5991.

The crafting recipe for anvils makes it totally clear that they should have iron sounds