I had something simular to this.
I could not rewoke the title from my direct vassal (count) as I was his King (and no dukes in kingdom).
The button was greyed out and explanation was that he was not my direct vassal, while he actualy was.
Seems like game get confused, since previously he was a vassal of neighbouring Duke and I attacked him while he was rebbeling and took him as my vassal. Also that helped me to form my Kingdom (while untill than I was the duke too). Not sure what of this two produced the bug, but, once when I turned off the game and reloaded it again, he acted fine (correct) as my direct vassal and I was able to rewoke his title.
Maybe this can be fixed.