Situation is me, Emperor of Hispania. Vassals are religious vassals: Prince-Bishops ruling each county outside my demesne, and a Prince-Archbishop above each. Wanting to make the Prince-Archbishop of Castille King-Bishop of Castille, Prince-Archbishop of Leon King-Bishop of Leon, Prince-Archbishop of Galicia King-Bishop of Galicia, Prince-Archbishop of Beja King-Bishop of Portugal, and Prince-Archbishop of Barcelona King-Bishop of Aragon. I hold the Kingdom titles for Castille, Leon, Galicia, Portugal, Aragon, and Andalucia (Crusade for Andalucia gave me that one). None of the Kingdom level titles appear attempting to grant the title to a vassal. Does this have to do with the fact that kingdoms these days require holding two duchies to create? Seems odd to apply that to handing out titles.
Edit: In a test where I revoked one Prince-Archbishopric and handed it to another Prince-Archbishop, the title still cannot be given to them