Baby zombies doesn't mount polar bears

A few years ago I heard that in Minecraft Pocket Edition, baby zombies can mount animals, including polar bears. However, the baby zombies no longer mount polar bears in the latest Bedrock Edition. What's reason?

Steps to reproduce

1. Continuously summon zombies until a baby zombies that can mount mobs is summoned
2. Summon a cow

→The baby zombie mounts it.

1. Kill its mount 
2. Summon a Polar Bear

 →The baby zombie doesn't mount it.