The people of Magical Almost-Earth needed the Chosen One to be brought from our world. Now that the day has been saved, Chosen One would have liked to stay, marry some royal sibling and enjoy life in a whimsical world where magic exists and people are grateful to have been saved.
Alas, this was not to be: something in Almost-Earth makes it uninhabitable for Earth humans. Maybe some isotopes are different, or the chirality of some molecules is inverted, or some physical constant is different?
Whatever it is, it slowly causes health problems that will result with death unless brought back to Earth. Symptoms should ideally become significant after a few years, but not yet irreversible if Chosen One returns to Earth at that point.
What could cause slow but inevitable and ultimately lethal health problems to a normal Earth human in this world?
Magic exists (part of the world's charm), but is out of scope in this question. Simply consider it as another type of technology in this context. Local medical science is in effect as advanced as ours: if we don't know how to prevent it, neither can they. They cannot sort isotopes or similarly high-tech, extremely expensive operations of the kind.
Earth is basically inaccessible, apart from a one-time round-trip for taking Chosen from (and back to) Earth. So they cannot simply, say, grab special food once a week from Earth to solve a particular health problem.
Similarly, local environment and ecosystem are similar enough for a human to survive without problems until those symptoms start. Some fauna and flora can even be shared or closely related. And the cause should not be obvious to an average human with higher education, until symptoms are starting. (It can be obvious to a specialist, with Chosen One simply not being specialist in this particular field.)
It must also be one-way only: Chosen One doesn't cause significant health problems to the inhabitants of Almost-Earth (though draconian quarantine and the local equivalent to vaccines and decontamination were applied to avoid a plague apocalypse).