From reading the article, it’s possible nuclear arms race, military tech transfer, and refugee crisis to name a few:

But Kim’s increasingly aggressive rhetoric against Seoul has the South once again debating whether it should have its own nuclear arsenal. North Korean troops on a Ukrainian battlefield would only further unravel Beijing’s plans.

…

Experts say Beijing is just as worried as the West about what military technology Putin might sell to Kim in exchange for troops.

“Satellites, for sure,” Mr Foster-Carter says. “But Putin is bad – not mad. Russia knows just as China knows that North Korea is a loose cannon. Giving [Kim] more technology for nukes is not a good thing for anybody.”

Experts believe Xi is unlikely to do anything drastic because China needs a stable North Korea – if he cuts off aid, he would likely have a refugee crisis at the border.