During the last turn Archmage used Armagedon spell which killed me and all my creatures including Wall of Reflections. Because of wall's ability Archimage health was reduced to 0 which means that we both lost. However, the game claims that I lost. I think, this is a bug. See the picture...

 Yea, it is quite annoying.  It's happened to me a few times and it doesn't even matter if his health is more negative than yours.  He still wins.