ES2 lets you reject force surrender term but you need to pay a lump sum of influence to do it. They need to bring it back. Give the player "Ruthless" badge for refused the term of surrender and give it something like making other neighbouring AI frown upon you and might make them alliance together to bring you down.

Note: The diplo screen says I'm the one forcing his surrender as opposed to him offering his surrender. At the same time the lower corner button where you'd normally end turn is instead blocked with "will you accept this offer?"

In other words, it's treating the loser's "offer of surrender" as if the winner were "forcing surrender". An "offer", by definition, is not mandatory.

Now I started this particular game around release and only got back to it after completing another game where I specifically avoided combat, so I'm hoping this bug is just some residual wonkiness from patches. (like how I can't see any harbors in this game)

Once I'm done with the game, I'll start a new one and try to force the bug again and then make a formal bug report if it still exists.