I have written about this before on Tumblr back in December of 2016 and pasted it to one of my Rob x Molly pictures in my gallery. But at the time I had written this theory, I hadn't seen every single episode of Gumball (although I had seen all of the Robs multiple times, minus The Ex because it hadn't been aired yet or even announced. But that doesn't make any difference in my theory.
But in order to explain my theory, I need to mention that I am not a believer in the Manhole Theory. If you are a tumblr user or just talk regularly to other fans of Rob, you may have heard of it. It's the theory that Rob found the Void after Darwin kicked him into the manhole in the episode The Pony. To me, this theory makes absolutely no sense because of two simple facts: 1. Rob appeared twice in Elmore after being kicked into the manhole, [Once actually inside the manhole and in example no. 2] and 2. Rob obviously couldn't escape from the Void on his own and it also greatly disfigured him when he did, but he appeared in The Finale, which happened several episodes after The Pony, and you can clearly hear him saying, "They kicked me into the manhole!" So no, I doubt that was when he found the Void.
And now, onto my theory. I may take a couple of breaks to explain why I think certain things so we can all be clear on how I got to a certain conclusion. So just bear with me and we'll get through this. You're more than welcome to criticize my ideas or disprove my theory if you find any holes in it, but I have put a lot of thought into this and I've done my homework so I am really unsure of any holes that I could be missing.
So basically, I believe that Rob found the Void on accident instead of being sent there, looking for Molly. In the episodes The Party and The Fight, Rob and Molly are seen standing together and partying together. Of course, along with Alan but he doesn't have much of a stand in this theory right now. In the episode The Party, Rachel Wilson, the host of the party, states that to get into the party you must bring a date. So Rob brings Molly. This at least means they are friends, or possibly even girlfriend/boyfriend. This is important because of Rob's extreme sensitivity to being forgotten or ignored, because in the episode The Void, it is made perfectly clear that once you are destined to be banished there the world will try to cover up any trace of you and your existence because you were deemed to be a mistake for one reason or another. We don't see much of Rob and Molly's relationship because they are not main characters also. [We may see more answers if Ben Brocquelet actually makes his Rob spin-off, but I doubt he ever will...]
I believe that Rob never forgot Molly when she vanished. The rest of the school did, and were slowly beginning to forget him as well. I believe Rob asked for help and people seemed to care, but once Rob left the room they most likely forgot that the encounter ever happened and Rob was left on his own again. I believe he did almost exactly what Gumball did and went to her house looking for clues.
This is where I want to give you an explanation. In The Void, it is proven by Gumball that the Void can be pulled open by bare hands, and also has a very powerful suction. Had Gumball been alone, he would have likely been sucked right in. But Darwin was there to pull him out in time. But Rob mostly was a lone wolf and got sucked into the Void.
This is also another situation I was to point out: Yes; Rob was definitely destined to be sent to the Void eventually. People were forgetting about him, his name, where he lived, who he was, everything. But that doesn't mean that Rob couldn't enter the Void before the Void had a chance to swallow him itself like it did to Molly. In the episode The Oracle it is shown that Gumball and his family are destined to be swallowed whole by the Void [most likely their fate when the episode finally ends, as it will then be considered 'obsolete'] and the idea is even further supported in the episode The Copycats, where at the end Gumball and Darwin both not only glitch for a moment, but they have a realization about it that makes them visibly uncomfortable. But Gumball has been to the Void a grand total of three times! The first time being in The Void, [We won't count The Nobody, seeing as it is only Rob's re-telling of the episode] The Disaster and The Rerun. So this is a very possible situation. It is very possible to visit the Void multiple times before being swallowed up by it against your will.
Another situation that backs this theory that Rob was sucked into the Void is the fact that he is able to move freely. Molly was frozen in her treehouse until Gumball touched her, completely unaware that time had even passed while Rob was stuck on his island screaming, running and waving his hands around. These points are proven in The Void and The Nobody. Also in these episodes, it is also proven that when Gumball, Darwin and Mr. Small entered the Void they were able to move and explore freely. Meaning that it is very likely that Rob was able to do the same. There was no one around to touch Rob, and it was very clear that he was having a hard time leaving that island.
I believe that when Rob came back to Elmore disfigured, he realized he failed in his mission to save Molly. I also believe that his memories of his family are true, that he did not have a mother of siblings despite having temporary amnesia. I believe his clothes are a strong indicator of his financial troubles [tiny shirt, tiny shorts and tiny shoes] and his anger toward specifically Gumball and even Banana Joe points toward strong insecurities. He has anger toward two people big big, loving families and girlfriends/friends. I believe Molly forgot about him once she lost her tin foil hat, but she has never said anything about him or made any move to talk to him. But in every episode to date, specifically Molly and Glitch Rob haven't even been seen together in the same episode at all.
In the episode The Love, Rob returns to Elmore to sing one line, "Love has no name." This could point toward his selfless acts to save Molly from the Void, but I mostly say this because I find it odd that they place him back in Elmore when he hasn't been seen in school since he became disfigured before or after the song. I am not sure exactly what to make of that song, I could just be looking too deeply into it but that's kind of the point of the theory.
I also find it interesting that in the episodes The Stories and The Rerun, the first being a Molly centered episode and the second being Rob, they have one thing in common: No matter how sad or angry they are, they can't ignore someone who give them selfless attention. They are both very lonely people who just want to be noticed and listened to. Molly took a hit in the face from a fast flying dodge ball for Gumball and Darwin simply because they listened to her and Rob couldn't bring himself to satisfy his hunger for vengeance once he realized that Gumball had come back to save him from the Void instead of leaving him to rot like before, even going as far as to try to be friends with him in The Ex. The fact that Rob is infamous for being angry at being forgotten even more points to the idea that he and Molly would be close friends and that he wouldn't forget her when she vanished.
I am honestly not sure why he hasn't come back to either Molly or Alan, but Rob has been near Alan during The Bus when he hijacked it with a bomb or, a 'ticking briefcase'. Nothing came of the situation, but it was proven that in The Void Alan had forgotten about Molly completely and maybe did the same with Rob and has moved on with his girlfriend Carmen, so maybe the three weren't as good of friends as we originally thought.
These are my thoughts. I hope I haven't left anything out. If you have any questions, ask and I'll definitely get back to you as soon as possible.