[Kristoffer O]

It might be that the AI spend less gems when he has a high income of gems, hoarding it for mightier spells. The AI is not so conservative with gems if he has many, but low incomes.

When you dispelled his gale gate, there was no use for him to collect gems anymore, instead he tried to recast his gale gate and other spells. Thus you suddenly got much more from your arcane nexus.

It might be bugged of course, but AFAIK the spell mechanics has been the same for a long time and I believe it worked in Dom I final Versions. Some Dom I vets can probably correct me if I'm wrong.