The bug:

Creakings cannot be ridden by any entities, neither using /summon with the Passengers tag nor /ride.

How to reproduce:

1. Execute the following command:

   /summon minecraft:creaking ~ ~ ~ {Passengers:[{id:"minecraft:illusioner",HandItems:[{id:"minecraft:bow",count:1}]}]}

   → The illusioner was not summoned along with the creaking.

2. Summon a creeper.
3. Execute the following command:

   /ride @n[type=minecraft:creeper] mount @n[type=minecraft:creaking]

   → The creeper is teleported above the creaking, but doesn't start riding it.

Expected result:

Creakings would be able to be ridden by entities using commands.

Observed result:

Creakings cannot be ridden by entities using commands.