The "RepairCost" tooltip shows up on every custom spawn egg [example: /give @p vindicator_spawn_egg{EntityTag:{id:"minecraft:illusioner"}}]

This is probably related to the renaming of the item as I can't reproduce your issue with the provided command but I'll change it so that a RepairCost of 0 will not be shown. This way, a renamed item should not show a RepairCost unless it has actually been, for example, enchanted.