Alpha bug (nearly instakill my whole team)

So i had a alpha battle, and this happened.
my whole team was nearly full HP at the begin of the turn, and one single toxic tile suddenly did this:

https://gyazo.com/1abc29af4fdac3f259a28ed903b32fd1

all dragons to 1% hp (and 1 dead)
has anyone else experienced this? or is it a lonely one time bug?

for that alpha, did you use a tile that was a gas tile?

A tile next to it.
Happened again recently. I discovered it’s a visual bug, when I tapped on my dragons they still had their HP. That one dead before must have been a unlucky alpha focus.