Ask Lemmygrad

Maybe its not widely enforced, and only if they don't have the proper documents. Since DPRK and China are allies, maybe China only sends them back if their hands are tied.

I would assume people without work permits and such documents would get sent back if caught by authorities, as that is generally a normal law in most countries. (Just my assumption; I'm not aware of the specific laws in China around this.)

The reduction in visa renewals for north Koreans working/residing in China is a result of (US-led) UN sanctions:

Article from 2017: "North Korean workers leave China because of UN sanctions."

More and more North Korean workers are being forced to leave China following the sanctions imposed by the UN on 12 September. Beijing has announced the closure of all North Korean companies and joint ventures within 120 days, and a clamp down on the issuance and renewal of visas. [...] The closure resulted in the departure of about 2,600 North Koreans from the northeastern provinces of Liaoning, Jilin and Heilongjiang alone, where workers are employed in the fishing industry. A photo showing hundreds of North Korean women waiting for repatriation is circulating on WeChat, a widespread messaging application in China. It is estimated that at least 100,000 North Korean workers reside outside the country of origin to earn strong currency, for an annual total of remittance estimated at around US $ 500 million.

Although China went along with the 2017 round of sanctions, in 2022 China and Russia vetoed a new round of UN sanctions on DPRK and proposed lifting some of the earlier sanctions from 2017, as they felt the US had failed to engage in its end of peaceful diplomacy with DPRK and therefore the sanctions should be reduced and no further ones should be imposed.

It seems to me that China would be and is fine with DPRK citizens living and working in China, but is forced into a diplomatic balancing act as they try to maintain peace conditions while the imperialists are on their usual mission to split up and destroy their economic rivals by any methods they can achieve.