This is a bug Frontier are looking into. When the star density becomes too high, stars start disapearing from the "sky box". It is not entended and you can see that effect before reaching sag A as you see low and high density star regions in the sky marked by very clear lines.
Really? Interesting, is that a fairly new bug or has it been around forever? I don't recall noticing this bug when I was last in the core over a year ago, but maybe I just didn't notice it either...