Did it worke for the peasants? Is there a spell mesage in the region?

That's maybe, because all spells I know increase the income for the peasants only. The workers salary is the salary workers of a faction will get. Peasants have a higher income even without fortification (13, if I remember right -> increased to 15 with spell and fortification).

The peasants' income hasn't changed, either; it's been a surplus of 2 per peasant since my first turn (so should be an income of 12, with the fortification I got starting as a human). The spell did not change that...

Are you sure that the peasants don't get the extra money? The money in the region is shown in the report, so knowing what you took from them you can calculate ow much money the peasants made. Maybe they got the extra money and the report just does not show the magic bonus.

Yes, I'm sure they didn't get the extra money.

I had a look the two effects stack, definitely. I consider it pretty unlikely that there is a bug in any of this. But I can take a look.

The peasants in this region get a wage of 13, checked with the debugger. Seems okay to me.