Is this a known bug? I wonder where all of Pisa's trade posts went and...they've become feudal. Within the current ruler's lifetime, too--he was part of a patrician family and was elected by the patrician system as you can see.
He has a strong claim on a title which means he could have been landed before. The first holding a character gets sets his government type so if he was first landed with a castle his government type becomes Feudal (this can never change for that character) so when he inherited that King title the government changed to Feudal (his government type). Check the history of that title he has a claim on. Note that the game does not tell you directly what government type a character has (if any) - every unlanded character in a court shows as that court's government type but if that courtier had been landed before then his government type could be different than what it shows.