This one is... odd. I use Annex quite a lot, mostly because we all know our minions are horribly incompetent. Here's the thing though, when I only have enough mana to cast Annex once (149-75 by default, lower with mana shrines), it doesn't do anything. Takes my mana but doesn't claim or destroy tiles. When I let my mana replenish the spell works fine until I drop below twice the mana cost. It's like the spell takes my mana before checking if I have enough.
This bug seems to be limited to Annex. Other spells work fine. I've verified the game files on Steam immediately before testing, yet the issue still occurs. I get this issue in the original campaign, my pet dungeon, skirmish and sandbox/scenario. Haven't tried it in the other modes yet, but I expect it will be the same there.
I'm running version 1.6f11 of War for the Overworld.