Oops. My mistake. You are right. I must have done something wrong...I won a war against a rebellious Duke, but I couldn't revoke one of his titles without incurring a penalty. I thought the game had changed, but apparently not. I was able to revoke the title of another rebel without any penalty after winning another war. I don't understand what happened with the first one though. Maybe I clicked on "white peace" instead of "enforce demands" ? The game says I have a truce with him (he's in prison, of course) and that I need a "special" reason to revoke one of his titles, which I do not seem to have. The button that revokes the title is greyed out.
Edit : Maybe I made him my regent and you can't touch the regent, even outside of regencies ? I'll check that out :/