One of my opponents claimed a city I assimilated some turns before. I refused it and they immediately declared war on me. They had war support of 7 and I had 2. We fought no battle at all as the war ended next turn as expected but with forcing me to surrender by passing over the claimed city. It's a joke, right?

Did you retreat perchance or did something else to drop your War support? The AI having actually declared a Surprise war is great news. :)

I reproduced the issue from auto-save. So initially it had 7 war support. However the tooltip mentioned that Refusing the Demand gives the opponent 20 war score. So theoretically they should have had 27 after refusing which made the AI able to declare suprise war. However I didn't see the war score number changing right after pressing the Refuse button (only the gauge got bigger on their sode) but they immediately managed to declare war. And I neither get how they managed to force me surrendering along with losing my city as well as paying reparation: did they have enough war score for such request?

Reproduced it again. So the tooltip is confusing. It states that the war support is 7/100. However at the bottom of the tooltip it shows an instant value of 28.

Anyway I checked using Surrender: giving the claimed city over took only 10 war support point.

So it seems the war resolution works well... and the AI is opportunistic :D

Yep, I thought it might be some misunderstanding with the interface or something. I am really pleasantly surprised that the AI took advantage of a grievance and actually followed through with it, both by declaring the war and moving in troops to occupy that territory. :)