In my game as Native Americans, I saw Ethiopia giving independence to an island they colonised. The weird thing was that the vassal was too Ethiopia, with the same leader. I suppose that they should give independence to a different civ, for example Mali or Arabia. But no, another Ethiopia.

I don't think that a save is necessary, and I don't even have a save from about a turn before releasing.

This happened to me too. In my late game I found an island that barbarians have had since the beginning of the game (I didn't even notice it). Anyhow, I took the island and colonized it and the new civ had the same nationality (American).