No, I've quite recently gotten Iberian wedding and took Naples under PU as well. It might simply trigger a break if the senior partner didn't have prestige / relations to maintain the junior at the time the senior itself is placed under PU.
Ah gotcha, this is the instance I was referring to, my last 3 games Naples was set free after Iberian Wedding, so I thought it was patched out. But seems to be other mechanics involved, I guess. Thanks.