The /summon command will not spawn mobs riding other mobs anymore.
I typed in this command:
/summon Pig ~0 ~0 ~0 {Riding:{id:Villager}}
I expected to see a Pig mob riding a Villager mob.

What actually happened was it spawned a pig.

Steps to Reproduce:
1. Enter this command into chat:
/summon Pig ~0 ~0 ~0 {Riding:{id:Villager,Riding:

}}
2. Press Enter
3. See the pig without its villager.

EDIT: this issue is resolved.
Correct command here:
/summon Pig ~0 ~1 ~0 {Passengers:[

]}