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.
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Comments 14

Confirmed for 1.13.1.
Can confirm in 20w51a.
Can confirm in 21w03a.
Can confirm in 21w05b.
Can confirm in 21w17a.
Can confirm in 1.17.1.
Can confirm in 1.19.
still in 1.19.4

Seems to affect 24w14a as well.
@Skylinerw mentioned to me, that this was likely because the the slimes now get spawned with a base of 20 by default, and when they first take damage, the game will notice the discrepancy and then revert back to the correct amount of hearts while displaying the difference in the "damage-hearts" visually.