If you use the "pick-a-block" button (mousebutton 3) on Mobs, you will get an invalid spawn egg.
Pick-Block on Mobs give normal Spawn Egg
Middle clicking a mob gives you a blank spawn egg
When middle click on mob, gives you white spawn egg
Incorrect Spawn Egg from Pick Block
Spawn Egg doesn't work when right click
Mouse button 3 on mob gives blank spawn egg
Pick block button on mobs gives blank egg
when clicking on a mob to get a spawn egg you get a blank egg named spawn
Middle clicking on entity (Mob) give "minecraft:spawn_egg 1 0".
Pick block doesn't work on mobs to get spawn egg
Pick Block (Button 3) on a Mob
Can't middle-click mobs for spawn egg, and placing a spawn egg on another one gives you two.
Mob Middle click
Shulkers give empty spawn egg when middle-clicked
Click with a spawn egg on an adult mob doesn't spawn baby mobs