I've seen this twice now happen to Navarra. I'm not playing as them but I see that when the king from the 867 start dies his son ens up with just the county and a weak claim on the kingdom. Nobody has the kingdom, it just killed itself. Why/how?
Because the King title has no active county inside it
Bug with gavelkind, it now gives kingdom level titles to characters without giving them any actual holdings. Hence, they become landless and their title gets destroyed.
It's pretty silly.