Spawn eggs can store all entity information in the EntityTag compound, in addition to the entity id. When used, the spawned entity will assume all stored nbt information. That is vanilla behavior and used to work just fine on Spigot.

Since the most recent series of patches however, said EntityTag gets wiped randomly. A spawn egg will lose all information beyond the entity id and show the same behavior as if it was just grabbed from the creative menu.

 

 

Steps to reproduce.

• Give yourself a spawn egg with additional NBT data, such as age.
• /give @p minecraft:spawn_egg 64 0 {EntityTag:{id:minecraft:cow,Age:-1000}}
• Watch the spawn egg produce a baby cow.
• Open your inventory, pick up the stack, and drag it over your inventory to split it into 2 or more.
• Watch the resulting stacks spawn normal cows. You can use debug tools to confirm that the EntityTag compound is no longer available.

I encountered other ways to trigger the wipe, but this is the easiest. For all I know, it can happen any time, randomly. This issue is present in Jenkins build 1087 (latest), but not 1082.