Why is the independence war going on, when I'm no longer his vassal anyway?

I was a Count under a Duke under the Basileus.

I had enough counties to create a new Duchy, but the game wouldn't let me, because "you can't create a title as great as your liege's".

I declared a war of independence against my liege. After a few days, the Basileus created the Duchy I wanted and gave it to me, so now he's my liege.

I assumed the war would end with a "casus belli no longer valid", instead it's still going on.

Is this a bug, or is it somewhat intended? What am I supposed to do?