Slimes are summoned with the wrong amount of health

After summoning in a slime (i'm pretty sure size doesn't matter) they seem to be invulnerable for the first hit.

If hit, they will indicate a strong hit, with a lot of hearts (I counted 8-10 i believe) showing up, even on the smallest ones (harder to see on the bigger ones since the hearts seem to be displayed partially inside them).

Steps to reproduce:

/summon slime ~ ~ ~ {Size:0}

punch it with your hand.

what i expected: the slime dies.
what happened: the slime displays a lot of hearts and survives, actually dying at the second hit.