Rowanne cast doesn't do damage against a low life enemy

Platform, device version and operating system
Steam/Windows 7

Screenshot or image
Video

What you were expecting to happen, and what actually happened
I expected Rowanne’s attack to deal damage, kill Xathenos on 1hp and win the battle, but after the attack animation enemy just got an another turn.

How often does this happen? When did it begin happening?
On this particular battle, I already cast Rowanne twice without it killing Xathenos. Recorded the third try. Skull attack did finish Xathenos after. I never had this particular thing happen, but it’s not the first Sunbird related glitch. Once in a while its kills glitch the remaining alive troops out in a way, mostly UI/placement wise.

Steps to make it happen again
Start a normal difficulty Groshnak Explore battle (get Xathenos?)
Cast fire bomb to do damage to some troop
Cast Firebird to finish all but Xathenos, leaving it on low life (1hp?)
Cast Rowanne.