Why wouldn't they? China and Russia are not natural allies and they both have an interest in maintaining influence over energy-rich Central Asia.

Russia invading Ukraine gives China license to further deepen its relations with its neighbours, without worrying too much about Russian blowback.

It's the same situation as when the US invaded Afghanistan and Iraq. China got a get out of jail free card in terms of ramping up military and establishing territorial claims in the South China Sea. No kerfuffle with the US, who were simply too deep into a quagmire to care.