Custom variant models of an item have troubles with rendering : the model renders everywhere, except on Armor Stand head, and when the model is on the head of an Armor Stand inside a mob spawner. Notice that the model renders on others mobs head and on a player head.

Here, the model is bricks, and the original item is the iron hoe :
Iron hoe json file : https://api.myjson.com/bins/t80of
Bricks json file https://api.myjson.com/bins/uh11r