Vanilla mob overrides do not use the default variables

Vanilla mob overrides do not use the default variables if the mobs spawned naturally.

For example:

SHEEP:   Display: "<caster.var.UiName> <caster.var.ParentPName>\n&c[&d<mob.level> &elevel &c<caster.hp{round=2}>&e/&c<caster.mhp{round=2}><&heart>&c]"   Health: 20   Damage: 3   Faction: Animal   Variables:     MobType: normal     UiName: "&6CoolSheepName"     ParentPName: ""   ...   Skills:   ...   - setname{name=<caster.name>;delay=2} @self ~onDamaged   - setname{name=<caster.name>;delay=2} @self ~onTimer:10 ?incombat{}

When summoned via an egg, a mythic egg, or a command — everything works fine. But when this mob appears in the world, its nickname shows as "UNDEFINED UNDEFINED [X level Y/Z]".