After Y-Dragon Head equipped itself to X-Head Cannon, it stopped being targeted by Rare Metalmorph. Similar examples can be seen with Fiendish Chain, Oasis of Dragon Souls, and Call of the Haunted.
This IS the expected behavior in other YGO games. Because Y-Dragon Head doesn’t actually leave the field, it becomes a union equip, Rare Metalmorph does not get removed. It simply continues to exists.