First of all, my HP did decrease.. 1 hp -
When you see clearly to the video of Fizz
First thing by which you can imagine that it's a glitch: He throws two nades, when you see clearly one stays on the ground but one disappears..
2nd nade disappears in the ground.. due to some interior objects missing, it goes down. Nade's damage is according to how much distance do you have kept. The nade's distance was far from me because it went under the ground somewhere and it was falling.. it hit me and my HP was decreased to 99.
Second thing to notice: Other nade explosion leave more flame than the second nade, which clearly shows that the nade is outside the interior.