I'm pretty sure that's what happens when they get negative mana. As in, they have a bunch of enchantments, but their shards gets razed by either a monster or other AI, and their mana got drained into negative values. Since the game doesn't keep track of it, it shows up as odd numbers (usually -2 billion, not sure why it's 600m in this case). Right now, I don't think there's any mechanic to remove those unmaintainable enchantments. So in a sense, it is affecting gameplay (they are keeping enchantments they shouldn't be), but it's not as if they have unlimited mana to cast. I mean, what did they use all those 600m mana on?

you can maintain enchantments, click on your mana icon at the top, this will bring up a list of enchantments currently active and their mana upkeep.  You can also get to thi sscreen by using the kingdom management screen, and the "enchantments" tab on the right.

The AI getting silly numebrs for mana has popped up for me a lot too.  It was more prevalent in .913 though.  I've finished about 3 games of .914, and only one of them had crazy mana total for the AI, and it was only one team.  (big improvement over .913 )