I had experienced an earlier issue with Axebane Guardian overproducing mana for the AI, but I couldn't reproduce it immediately and didn't have much time to devote to it.
Out of curiosity, was this game 1, 2, or 3 of the match? Had there been a previous state where the AI had more defenders out? I'll see if I can figure out exactly what's happening and hopefully write it up again with enough specificity that one of the devs will know what's going on.