There's an issue with gondor's transition from Denethor to Aragorn. You don't get your levies back from Denethor and the game thinks you're not independent or a tributary/client state (can't propose a protectorate to Rohan for example). I was at war during the transition (both games I tried), one with sauron, one witth saruman.

It's like the game still thinks you play as Denethor in some aspects. I have a save if you need one and maybe an autosave right before the transition.
playing on 1.11.3