Why isn't my data merge command working?

Question

I'm trying to make a repeating command block that makes an arrow have an entity riding it when shot (I'm using a pig here as an example) but for some reason it says it modified the data, but it isn't working. Can anyone help please?

My command:

data merge entity @e[limit=1,type=minecraft:arrow] {Passengers:[{id:pig}]}

Answer 1

Unfortunately, you cannot modify/merge an entity's Passengers tag after being spawned. You can only modify this tag when the entity is being spawned.