I think it's just something to do with the asset population "catching up" with the player's location. Even though it doesn't involve any network traffic AFAIK, it feels similar to how it sometimes takes a while between the discovery scanner honk completing and the "bodies discovered" message and HUD display updating. I had one earlier, while scooping very close to the corona, that didn't draw the exclusion line until I'd more or less completed scooping and was about to engage the FSD.
My feeling is that the game just gets bogged down doing "other things" and that calculating and displaying the boundary is way down the priority list. I've been playing since alpha and seeing this intermittently ever since we've been able to jump between systems. I've yet to come up with a way of reproducing it on demand. It's just one of those minor things; nice if it gets fixed, not a game-breaker if it doesn't.