To be more precise.
I am playing as the King of Greece. I successfully declared independence from the Byzantine empire and I stripped it of all its Greek territories. I am going to take over 80% of the empire's territories and I will usurp the Title.
I was able to choose the Elective Monarchy succession law, since it's my favorite, and I chose the second of my four children to succeed me. God blessed my dynasty with four males and the second was born with the genius trait due to his lovable and smart mother.
I decided I needed more gold to hire more mercenaries, so I am going to convert the just conquered Isle of Crete to a republic. I create the mayor of the main city, I promoted him to rule over the county, then I assigned him the tile of Duke of Krete. That turns him into a Doge and gives him dominion over the whole island.
As soon as I turned my mayor into a Doge, the game warns me that I still have to select a successor. What happened? I go to the screen with the intention to select again my beloved little genius prince, but I can't find him in the list. I though, perhaps he's dead and I didn't notice XD. No he was there, at my court, healthy and smiling awaiting to find a proper bride.
I returned to the screen and I tried again, but nothing to do. So I said to myself, WTF? I reloaded the last saving, I appointed as successor my most annoying vassal, the Duke of Aegean Islands (I would kill him right away, but he's my father in law and I don't want to upset my beautiful wife), hoping that the bug takes him out of the way from the succession list. I create the republic but he's still there as a successor. My son, however, can be elected.
Ok, I thought, I want to see if I change a few conditions, that happens again. I reloaded the save game before the creation of the Cretan republic, I kept my son as successor but I married him, and I made him Duke of Nikaea. I created the republic. He vanishes again form the list.
I, again, reloaded changed again the successor to someone else and I created the damn republic. After that I re-elected my son as a successor and I kept playing.
Is that accurate enough now?