If you use a spawn egg that has a custom EntityTag NBT tag on an adult mob, it will just spawn the baby variant and ignore the custom NBT.

How to replicate:
1. Give yourself a Spawn Egg with custom NBT, like this:

/give @s minecraft:villager_spawn_egg{EntityTag:{CustomNameVisible:1b,CustomName:'{"text":"I am a Villager"}'}}

2. Using the Spawn Egg on a block will result in a villager with the name tag 'I am a villager!'
3. But using the Spawn Egg on the villager will result in a baby villager with no name tag!