i don't think it's a widespread problem, but there have been a few people here who have had problems with their steering wheel who swear they just used mild soap and water to clean the wheel (and one who's was obviously just scratched with something).
it doesn't solve your problem with getting toyota to replace it under warranty, but i did notice cartrimhome has a custom cut steering wheel cover:
Free Shipping Hand-sewn Soft Leather Wear-resistant Steering Wheel Cover For Toyota 4Runner 2014-2019