Anvil item entities get deleted when trying to stack with another anvil item entity. This only happens when both item entities are created by an anvil falling onto the same non-full block. Making them stack together afterwards by pushing them with water, or otherwise throwing an anvil item entity in the same block space does not produce this issue.

It looks like the anvil checks whether or not there is an anvil entity already, and if so, simply replaces it

To reproduce:
Place a bottom slab with on top of that a block, and stack multiple anvils on top of each other on top of this block. Then break the block, and let all the anvils fall down and turn into items.