It's accidental, the deadly doses are so low that just not separating them perfectly when weighing or packing might contaminate the other.

A likely explanation is that China is using the cartels to wage a 21st century opium war. (The obvious link to China being that China produces most of the chemical precursors to fentanyl, and until recently produced most of the fentanyl itself.) The average American drug user isn’t the customer; he, or rather his eventual death or disability, is the product.

More parsimonious explanation is that the PRC bureaucracy isn't going to waste resources cracking down on precursor producers for legal medicine ingredients when it doesn't cause domestic issues, or issues in the majority of countries, including other eight nation alliance members that have exploited China far more than the US. Like why wage 21st century opium war against only 1/8 of historic grudges. The primary incentive to assist the US med system driven opiate crisis prior during periods of better relations was because it was politically worthwhile to waste PRC resources to cooperate on US war on drugs. If PRC really wants to be nefarious, she would engineer less lethal drugs that keep victims alive longer to drain more American resources. And she would move back to actual drug production instead of maintaining olive branch precursor production. In so much as working with cartels on state level, IMO PRC benefits more from intelligence / influence of triad + cartel relations as they try to make headway into the Americas. The actual killing Americans is incidental because Americans will reliably find other ways to OD.

the cartel, albeit advanced, are basically a village idiot on the ground for the PRC. this all makes sense.

Fentanyl deaths are rising in many Western countries, not just the US. Plus, the US is the primary geopolitical rival to China and hence the most relevant target. It's not worth the effort for China to exact revenge against, for instance, Austria and/or Hungary because neither of those countries are relevant to China now.