Not sure if a bug or intended but it seems you can no longer move your capital to planets outside the cire sector (2.6 and 2.8) as reported here in the console forum:
Thread 'How do you cahnge the capital of your empire?' https://forum.paradoxplaza.com/forum/threads/how-do-you-cahnge-the-capital-of-your-empire.1478488/
Thread 'How do you cahnge the capital of your empire?' https://forum.paradoxplaza.com/forum/threads/how-do-you-cahnge-the-capital-of-your-empire.1478488/