That too. At least Gumball and Molly interacted a lot. So basically he wouldn't remember someone he never remembered when he barely remembered someone who was there every day. The presence of her missing weighed on Gumball and his mind was erased by the Void as well. Rob he barely noticed when the Void didn't erase him. Though it seems like one of those "sure, we could easily explain it, but then the episode would have no story" deals. Plus, it was at Gumball's insistence he was the villain.
Funny thing about Rob is, well...this is how he originally looked:
But somehow it was either easier or cheaper to animate him in 2-D, especially since he was no more than a background character anyway. So he was redesigned to:
So his very inclusion in the Void to begin with was an inside joke about how they just couldn't do anything with the character. They managed to take that one inside joke and make a major recurring plot point/antagonist out of it. That's why this show is so great. The level of commitment to an inside joke that they didn't need bother with.