When summoning an area_effect_cloud with WaitTime:0, it takes about 6 ticks for the cloud to apply its Effects to a player/mob within its radius.

AECs with WaitTime:0 and Duration:1..5 won't apply the effect before they despawn.

How to reproduce:

• Summon an AEC with WaitTime:0 and Duration:1..5. This cloud will not apply its Effects to a nearby player/mob.
• Summon an AEC with WaitTime:0 and Duration:6. This cloud will apply its Effects to a nearby player/mob, just before it's Age reaches 6 and it despawns.

Examples:

/summon area_effect_cloud ~ ~ ~ {WaitTime:0,Duration:5,Radius:10,Potion:"minecraft:weakness"}

won't apply the effect, while

/summon area_effect_cloud ~ ~ ~ {WaitTime:0,Duration:6,Radius:10,Potion:"minecraft:weakness"}

will.