Cannot reproduce in 18w19b
Still not fixed in snapshot 18w19a.
Unfortunately this is wrong. 18w16a changed the way coral blocks die (unless it's a bug, but I doubt that).
How to reproduce:
1. Choose a coral block color.
2. Place the coral block above water.
3. Wait a few seconds. Break it right before it dies (may take a few attempts because it's random).
4. Place the same color coral block where you broke it before.
5. You expect the coral block to die after a few seconds, but instead it dies almost instantly.
Confirmed for 15w38a
Sorry, couldn't find that one. This is indeed a duplicate.
It looks really weird for some reason.
I can confirm this.
A Guardian is a new mob in Minecraft. It was just released
Confirmed for 1.18-rc3