The real question is why Taiwan needs to source their Sky Bow missile from China. That's embarrassing.
First, there are three different types of Sky Bow missile, not just one.
Second, Taiwan isn't sourcing any of the missiles from China. The design is indigenous and most parts are bought from elsewhere. A few businessmen thought they could make a quick buck by outsourcing some of their work to Chinese companies and pretending the parts had come from a different source. But they were found out last year by NCSIST.
Well of course the PRC will sell them inferior material. Imagine the quality of material the US would supply Russia if Russia purchased components used on strategic weapons systems from the US.
The US either allows sale of items used in military equipment or it does not. It doesn't have a policy of selling bad stuff to screw over other countries.
It's also improbable what the Chinese producer(s) knew what the components were needed for, because they'd be in serious trouble if the Chinese security services found out they were supplying even indirectly the Taiwanese military for something like air defence. Even asking permission to make deliberately bad parts would be a huge red flag, because the CCP would not know how defective the parts really were and whether they might end up being used somewhere in China - or damage China's reputation in export markets.
It's also highly unlike Chinese supplier would produce something specifically for the contract. More likely is that these were items available for general purpose on the Chinese/international market that were just low quality but cheap, hence why they were used. It's also why the middlemen had to change the origin of the parts. If the Chinese producers were part of the fraud, they'd have done it themselves.