I have confirmed this glitch by experiencing it twice.

I was building a staircase in the Nether, and took some minor damage from a Blaze fireball.

I died instantly.

I've had this happen to me twice. I had full Protection 4 diamond armor with a Notch Apple's regeneration going.

So, in short, standing on staircase, hit by blaze fireball, dead at 100% health instantly.

PLEASE FIX SOON!

-Bill