As an aside to the original question, for me there is a difference between "to put on weight" and "to put weight on".
The first case implies that the subject and object are the same person. "I've been putting on weight" = "I am the one who is heavier now."
The second case sounds much less common to me. It implies the work of a farmer or animal breeder or anxious parent: Person A provides fattening food for Person-or-Animal B.
I'm guessing that the original "on" was the first one, and that, on re-reading, the writer thought: "Ah, it sounds as if the farmer is gaining weight. I should move that 'on' on a bit further, to avoid ambiguity."