Like hearing and balance are both in the same sensory organ, and consist of sensitive hairs in liquid-filled chamber? Or the pancreas, which consists of separate endocrine and exocrine glands in one wibbly wobbly visceral organ? Nature would never do things like that.
I don't agree hearing and balance are the same sensory organ. They are two separate organs, just very near each other and indeed resembling each other, which explains why they are near each other.
The thought I was expressing is that there seems to be no separate organ at all here and that seems unexpected to me. I would expect there to at least be something like a specialized region. Not just that the eye in general happens to also have this ability.
I wouldn't say "suspicious", but I do think it's interesting. Yes, it needs to receive light, but any exposed part of the animal could do that. Does it "feel" to the animal like it's part of vision, in the way that we experience taste in an integrated way combining signals from taste and smell receptors? Does pairing with vision make it easier to process both together for navigation? That is, you're point is how a separate organ would make it like an independent sense; is it functionally beneficial for it to not be independent?
This kind of magnetic sensitivity seems to be dependent on receiving light, because the sensitive molecules form the radical pairs when exposed to light. With this information, evolving within the eye scenario sounds plausible to me.