Slowness effect on summoned mobs with /summon doesnt work

Slowness effect on summoned mobs with /summon doesnt work.

/summon Chicken ~ ~1 ~ {ActiveEffects:[{Id:2,Amplifier:127,Duration:99999999,Ambient:0}]}

Mob, position and Ambient are irrelevant.

But if you throw them a splash potion of slowness after summoned (slowness I or II), they become still.