TL;DR
It seems China has more to gain (or fear by not) by complying with Western sanctions than it has to gain by aiding Russia in the latters time of need.
Overall, making Russia happy is worth less than the ability to trade and work with the the rest of the world, long after the dust has settled in Ukraine...
A view from the west, but could include clues as to why:
https://supchina.com/2022/03/04/the-economic-fallout-of-russian-sanctions-on-china-explained/
China’s exposure to the Russia sanctions, explained
China’s direct exposure is small: While China is Russia’s largest trading partner, Russia is not even in China’s top 10. Bilateral trade will continue as well, since Russia’s trade surplus vis-a-vis China ensures that Chinese exports keep flowing until trade nets out.
Chinese businesses and banks appear reluctant to transact with Russia for fear of secondary sanctions.
Two of China’s biggest commercial banks just restricted financing or purchases of Russian commodities.
SMIC, which supplies chips to both the West and Russia...(carefully) complying with the Western sanctions regime would be much safer for the Chinese chipmaker’s bottom lines.
Chinese businesses cannot help but take Western sanctions seriously for fear of being next on the sanctions list.
https://www.washingtonpost.com/world/2022/03/02/russia-economy-sanctions-china-support-ukrain
China not emerging as lifeline for sanction-slammed Russian economy
Despite a close partnership built around mistrust of the United States, Beijing is hesitant to incur Western wrath for evading sanctions on Russia.
Beijing appears to be holding back over practical constraints and fears of secondary sanctions on Chinese institutions.
Chinese officials and commentators have urged caution at drawing American ire and instead suggested China focus on building up its resilience against similar measures that could target the country in the future rather than helping Russia now.
China’s businesses and banks appear to be seeking to avoid being pulled into Moscow’s standoff with the West.
Without making public statements, some Chinese institutions appear to be quietly adhering to sanctions, and there are few signs of significant attempts to create a lifeline for the Russian economy.
China’s announcement in February lifting restrictions on Russian wheat imports received a lot of attention, but it was just the fulfillment of a pledge made much earlier after concerns about the presence of dwarf bunt fungus found in previous shipments had been resolved.
For Chinese technology groups, undermining business relationships with the West to build up ties with Russia is not an attractive trade-off. For many of China’s leading technology companies, business with Europe and North America dwarfs that with Russia, especially when it comes to purchases of cutting-edge equipment.
“China would risk losing access to technologies from the West if it ignores restrictions on Russia. And the West has far more to offer than Russia in terms of semiconductors, software and high-end industrial goods,” said Dan Wang, a Shanghai-based technology analyst for research firm Gavekal Dragonomics.
China has the ability to aid Russian banks to make cross-border transactions using its yuan-based Cross-Border Interbank Payment System (CIPS), an alternative to the SWIFT messaging service to which the banks have lost access under Western sanctions.
But CIPS is limited by its current scale and Chinese restrictions on convertibility of the yuan, which accounts for a tiny portion of cross-border transactions compared with the dollar. On Tuesday, CIPS handled 11,513 transactions compared with around 40 million messages per day for SWIFT.