Still not solved on 1.134!
Bug also happens on other claimant quests like Lady Isolla of Suno, etc.
I believe this happens when the old king is in jail, either from your rebel faction or other faction, when you complete the claimant quest.
The new king/queen seems much less inclined to give fiefs to anyone else but the old king, even if you conquer a castle all by yourself and without the help of anyone, won't give to you but to the old king.
Please Taleworlds take care of this bug on the next patch if you can. It's happening since 8 months ago and was still not solved. Seems to be as simple as:
Finished claimant quest -> Check if old king is in any dungeon of any castle/town -> if he is -> set him free and exile him from Caldaria.
I don't even take claimant quests because of this bug... And they seem a pretty good early-to-mid-game way of having fun.