When using the command "/kill @e[type=Shulker]", shulkers that have been spawned in using spawn egg are not killed, giving the message "That entity cannot be found". However, shulkers spawned in using the "/summon Shulker" command are killed correctly, and exiting & reloading the world allows egg-spawned shulkers to be killed with the command.
To reproduce:
Use a Shulker spawn egg to summon a shulker.
Use "/summon Shulker" to summon another shulker nearby.
Use "/kill @e[type=Shulker]", only the second shulker will be killed.
Close then open the world, re-type "/kill @e[type=Shulker]" and the second shulker can now be killed.
This effects all Mobs that are spawned with spawn eggs!
Its impossible to select those mobs in commands with the target selection argument @e.
>>Error Massage: "That entity cannot be found".