Finally, I’ve found the cause!
Normally, Arch Magi reduce 2 mana points. But part of WoG is stack experience module. It’s fine, with it game is harder than normal. But, Arch Magi at 9th stack level reduce 3 additional mana points, and at 10th level reduce additional 4 mana points. So overall they reduce 6 mana points and that’s why you have negative mana points.
Solution for now? Just delete arch magi from army or reduce stack level with hiring new unexperienced arch magi.
But such thing can be treated as bug and would be fine if some of the VCMI developed add cap that can’t allow to have negative mana points…