Well, nominally the Kingdom of Lithuania. Founded as a Grand Duchy in the 12th century, one of the most powerful Dukes, Mindaugas, received baptism and surrendered some lands in the west in return for being crowned as King by the direct intercession of Pope Innocent IV; the Pope got a Christian bulwark against the Mongols, and Mindaugas got his crown and time to consolidate his rule against his rivals, as well as peace with the Crusader states. When Mindaugas reverted (or seemed to revert) to paganism, the Kingdom reverted to a Grand Duchy and the Crusader states moved against him. Vytautas the Great tried to regain the crown in the early 15th century (near the end of the game's time frame), but Polish intercession prevented the crown from arriving in Lithuania before Vytautas died.
Specifically, however, it shouldn't be that a creation of a King title occurs and the Pope forces it to revert. Rather, it should be that the creation itself requires a Papal blessing to occur at all, such that withholding this blessing means it never occurs in the first place. That's how it usually worked; in the example above, Lithuania wasn't a Kingdom until the Pope ordered the Bishop of Chelmo to crown Mindaugas. Mindaugas couldn't style himself a King until that occurred, though he was the one who initiated the process by approaching the Livonian Order. Basically, the Catholic player would claim the Kingdom title, but they wouldn't become a Kingdom until the Pope or some other major power (the Holy Roman Empire) accepted it. On the other hand, other religions, lacking such a centralized focus, wouldn't necessarily need such an extra step, and I can see the "Grand Duchy" title of Lithuania as still being equivalent to a Kingdom-tiered title in practical game terms.
Just nitpicking, but IIRC wasn't Mindaugas successor the one who reverted to paganism? And Poland had periods early in game , when not all Polish High dukes managed to get a coronation and thus didn't became king. Or that before Ottokar I managed to make the title of king of Bohemia hereditary by good political maneuvering, it sometimes was granted to the dukes of Bohemia as a personal title by the Emperor.