Here, there is no gap preceding the word vayelech, and the masoretic note (I reconstruct it here, based on sevara) on the side states that man deamar devayelech reuven it beih piska ta'ei -- those who say that there is a gap before Vayelech Reuven are making an error.
I first saw a discussion of it in Minchas Shai, who writes:
וילך ראובן אין כאן פיסקא
כי בפרשה זו אין פיסקא כלל כ״ש בריש סדרא וכן כתיב בספר חגי ומאן דאמר וילך ראובן בימי קציר חטים פרשה טעי:
I am not certain I understand this fully. Isn't the beginning of the sidra by Vayeitzei Yaakov. What is meant by kol shekein? Especially if Minchas Shai maintains that there is a gap at the beginning of the sidra.
Or Torah writes:
וילך ראובן אין בו פיסקא
כלל בכל הספרים גם רמב״ם לא מנה
That is, all sifrei Torah / manuscripts he checked didn't have it; and where Rambam lists the petuchot and setumot (based on the Aleppo Codex), he does not list this one.
But this does fit in with the other discussion, of whether there is an initial gap. Minchas Shai, following Or Torah, explained the statement that Vayeitzei is setumah as referring to internal gaps instead of the initial gap. But if so, there cannot be any internal gap.
When evaluating the likelihood of some variant being original, one thing we might assess is the likelihood of a change in either direction. Lectio difficilior, the law of the more difficult word. In this instance, is it more likely that someone accidentally missed a gap, or that someone accidentally inserted a gap?
I wonder how an accidental insertion of a gap could occur. I would expect making a petucha or setuma would need to be deliberate. (It depends on the metzius, and what actually happens in the copying of manuscripts, but perhaps if there were some space left on a line by accident, another scribe could mistake it for a petucha?) On the other hand, if the petucha or setuma were present, it seems quite possible to forget to copy it over. Also, the statement that Vayeitzei is setuma might also work to eliminate a legitimate internal gap -- particularly if this were a machlokes about an initial gap, but someone attempted to harmonize by casting the statement about being about internal gaps.
Laying aside any masorah about internal or initial gaps -- In terms of content, what makes sense? We have here a bunch of children being born. And suddenly, a fresh action by a new character. I can understand the assumption that there should be a gap here, as this begins a new topic ("parsha"). However, in the other direction, this is just the lead-in to another child being born. And so, really this should not be a new topic.
At the end of the day, I agree that it is most likely that this internal gap was a mistake.