Weird, looks like there are more bugs relating to the Sectors menu: When the Sol sector is selected, attempting to move the Sol system out of the Sol sector via the arrows does nothing and gives no feedback while double clicking will give the prompt that the original system cannot be removed from the sector. And when I go to the Lalande sector and attempt to add the Sol system via the arrows, I get nothing again.
That's all as expected, but when I have the Lalande sector selected, double clicking the Sol system somehow manages to add the system to the Lalande sector while simultaneously removing it from the Sol sector. Not that I'm complaining, as that's exactly what I wanted, but there's definitely some odd behaviour in the Sector menu.