How would science deal with a leg that grew back?
Given that this actually occurs in many, many species (including vertebrates) and science has studied the mechanisms for years then this is hardly news and science has been 'dealing' with this for decades.
Now of course I guess you meant in a human - but failed to make that point - and that demonstrates your human-centric approach (something that science doesn't suffer from). Were this to happen in a human, and I suspect it would be a partial occurrence rather than a fully formed limb - then science would look to what it knows about the mechanisms in other species with a hypothesis that exceptionally unusually similar mechanisms had been triggered. Worth knowing (which I suspect you don't) that the basic pathways and molecules that drive limbs regeneration in other species are also present in humans although their capacity for regeneration seems to be attenuated for reasons we don't really understand yet.
So in regenerative medicine (a field of study I contribute to) the question is more about why limb regeneration occurs in some species but not others despite having similar fundamental mechanisms to control limb growth. And. of course, whether it might be possible to stop the attenuation of limb regeneration in humans in a controlled manner.
So I imagine if a human leg grew back it would be because scientists (using their knowledge of limb regeneration in other species) had worked out how to trigger those mechanisms in humans. So most likely it wouldn't be a 'miracle', but a miracle of science.