I created a custom zombie spawner with a custom potion effect(speed 1).
But when a zombie spawned he got the potion swirl but the effect itself(speed boost) not.

What I expected to happen was...:

The zombie getting both the speed boost and the swirl.

What actually happened was….:

The zombie only got the swirl effect.

Steps to Reproduce:
1: Create a custom zombie spawner(also tested with other mobs) and give it speed boost.
2: Let it spawn a zombie.
3: Let the zombie follow you.
4: Notice the swirls and the lack of speed boost

It seems only to happen with speed and slowness.

I think it has to do with the attribute modifiers.
When you splash a mob with a speed or slowness potion it adds an atribute modifier to the generic.movementSpeed.
This doesn't happen when you spawn a mob with a spawner.