Setting a Entity HurtByTimestamp data to 0, doesn't work the first time

Setting the Entites HurtByTimestamp data to 0, doesn't change the data the first time it's executed.

Here's how to recreate.

Repeating cmd block

/entitydata @e[r=5,type=Zombie] {HurtByTimestamp:0}

Chain cmd block

/say hi

Other Example

type in chat

/entitydata @e[r=5,type=Zombie] {HurtByTimestamp:0}

type in chat again

/entitydata @e[r=5,type=Zombie] {HurtByTimestamp:0}

I haven't noticed this happening for any of the other tags.