First get rid of the loop, it should be on monster created, create health bar. The on created condition picks the monster that was created. Check if that sorts it..
Also the set position logic is a bit off because it doesnt pick a monster. You can try linking the bar and monster with unique IDs, or a better way is to use a container. This puts the health bar and monster together in a group so when you create the monster, it creates the health bar (no need for any create logic then) and also when you pick health bar to set position to monster they are then related/linked in the form of a container.
Also if you pin the bar to the monster on creation then it's easier to control than set position every tick.
Your final logic using a container would be something like :
Monster on created : set bar to required monster position, pin bar to monster.