Two more questions about this:
1. If your create a bishopric with your ruler's heir as bishop, what happens when he inherits the main title? Does the bishopric revert to an ordinary county?
In my current game, I created a bishop with my bastard son as bishop, and he immediately declared himself my legitimate heir 'based on the strength of his rule' so I now have a bastard bishop as my heir.
What is even more weird is what leads me to the next question.
2. Can bishops switch allegiance randomly with no warning? Less than a year after I created the bishopric, I clicked on the province to see how he was doing, and found that he is no longer my vassal but a vassal of the Seljuk Turks! There was no war, his loyalty was at 100% and rising last time I checked, and there was no announcement of a vassal declaring independence or anything like that.
It's quite strange that even if he was going to change allegiance that he would choose to pledge to the Turks as they are nowhere near - the province is in the middle of Spain, surrounded by Christian rulers, though many of the provinces are still Muslim.