ShulkerBullet cannot target AreaEffectCloud.
Explication:

When using

/summon ShulkerBullet ~ ~3 ~ {Steps:1,Target:{X:10,Y:4,Z:10,L:1l,M:1l}}

with the correct coordinates and the correct UUID, you can summon a SulkerBullet targeting the entity written in

.

But if the entity is AreaEffectCloud, it doesn't track it. It works for other entities.

How to recreate:
1-Do

/summon Creeper 10 4 10 {UUIDLeast:1l,UUIDMost:1l,NoAI:1} (near x:10 y:4 z:10)

2-Do

/summon ShulkerBullet ~ ~3 ~ {Steps:1,Target:{X:10,Y:4,Z:10,L:1l,M:1l}}

The shulkerBullet should target the creeper
3-Kill the Creeper
4-Do

/summon AreaEffectCloud 10 4 10 {UUIDLeast:1l,UUIDMost:1l,NoAI:1,Duration:10000000}

It's like when you summon the Creeper, but now it's an AreaEffectCloud
5- Do

/summon ShulkerBullet ~ ~3 ~ {Steps:1,Target:{X:10,Y:4,Z:10,L:1l,M:1l}}

The ShulketBullet should not track it.

Thanks for your comprehension, comment for help if needed.