Not gonna lie I've never heard of that happening.

It does happen sometimes, for various reasons.

I think my gray was probably caused by nutritional issues or inflammation, could also be stress/hormonal as someone else mentioned, so addressing that seems to be reversing graying. I was always pretty careful, but I've almost completely cut out seed oils now, as well as American wheat (almost always fortified/enriched with fake vitamin B that can be hard to process), and I've also been experimenting with hydrogenized water, which I think helped clear up inflammation.