When you respawn, there is a slight delay until you can move again, and I think that is a bug of itself. While you are in the slight delay after respawning, mobs and players can attack you, giving them a few hits until you can fight back.

Steps to reproduce:

• spawn a hostile mob near your spawnpoint.
• go unto survival, then do /kill.
• notice when you respawn, you can be hit even when you are in the slight delay.

I think that delay after respawning might be it's own bug, but I'm not sure.