Hi UnderCiv!

Do you have a save from the turn you ran into this, or the turn just before?

Save files by default are located: Users\[user]\Documents\Humankind\Save Files

auto-save 55.ctr

IMO: the whole forced surrender option needs a revamp.  I am not satisfied with the current manner that options are presented based on war support in the surrender.  The idea that I cannot keep a territory I captured and control w/o enough support is pretty poor.  That was the entire purpose of my war - to gain that territory, which they claimed out from underneath me.
Moreover, the idea that during war, I cannot implement any control over the captured city seems a bit absurd. I should at least be able to control the population effort direction. Not being able to shift population into the 'manufacture' category from money-production is a dirty-trick. Of course stability is low in that area if none of the population is allocated to that category - that is how the mechanic is established. What is the logic behind this fault? I don't see it. If I have an army in control of the city, I should be able to direct its resource allocation at the least.
Moreover, I had disbanded one of my armies, during the war, for this purpose of ensuring that I could have a population providing stability. If your not going to allow me to actually control the city - why did you allow me to make this sacrifice? That is a horrible setup on the mechanic of disbanding troops in a city during the war.
If I sacrifice a troop to a captured city during a war, I should gain support and stability in that city - making it more likely to side with my civilization - then at the time of forced surrender, I should be able to keep the territory if there is a balance within that territory for me to hold on to it. It should be its own support mechanic, not based on the entirety of the population support.
Or alternately, remove captured territories from the need of war-support in its entirety. It really should be that way anyway. And then the stability mechanic can be used for if it mine to keep after the war has been resolved or not. If it will shift its allegiance or not.