The consistent attribute when it comes to which rights are where comes down to defining and important stories. Silver Surfer is not a member of the Fantastic Four, and has teamed up with everyone, is a part of the Defenders, and so on, but all of his defining and most important stories are with the Fantastic Four. Black Panther premiered in and has been a member of the Fantastic Four, however his defining and most important stories occur with the Avengers and solo, so his rights are separate.
That's why Scarlet and QS are split, because they have important defining stories both with the Avengers and with the X-Men.
Because at the end of they day, the purpose of buying rights is not to have a slice of a non-existent universe, the purpose is to be able to tell contained stories on film, and so rights are split up based on storytelling, not on technical associations.
In this case, all of Uatu's most defining and important stories are with the Fantastic Four, so it's pretty likely that he's with them.