Mobs with no AI can target other mobs under certain conditions.

How to reproduce

Summon a zombie with the nbt 'NoAI:1b'. Run 'execute on target run say hi', and notice that there isn't a target. Then attack another zombie (that's nearby) in survival mode. Run the same command within 5 seconds and discover the mob with no AI now has a target.

What to expect

The mob with no AI should not be able to acquire a target because it has no AI.

 

I know this also affects iron golems.