After respawning the Ender Dragon and defeating it, a second End Gateway spawns. The original Ender Dragon must be defeated in order to obtain this bug, along with a respawned Ender Dragon. This bug will likely spawn another End Gateway if another respawned Ender Dragon is defeated.
UPDATE:
This will spawn a maximum of two End Gateways. Tested with a third Dragon and did not spawn an additional one.
This isn't a bug