I have found another possibility for the planets to flip.
I'm alone in my sector. When I was able to go into the next sector, I met the Corporate Sector. Later, I don't know how long it took, 3 of their planets flipped to me in just a few turns. Two were core worlds that became colonies, and one was a core world that stayed a core world. I didn't see any notification and wondered what was going on as I had already seen this post. The governor of the planet that stayed a core world has a loyalty to me of 70. That planet also is 37% in rebellion due to influence, so it's going to be interesting to see what happens.
Later, I realized I had send a diplomat to them. When I checked the diplomat, I found this.
That is probably the reason these planets flipped to me.
I found another candidate with that attribute, so I hired him and sent him to be a diplomat with another race. It has been just a few turns, so it may not be enough time for any planets to flip. I may have try this more often. I also may have to be very careful if the AI decides to try this on me.