Yeah, it definitely needs some explaining! But my interpretation is that Shen Wei found working with Zhao Xinci unpleasant enough that he gradually stepped back further and further to avoid interacting with the SID, until just around the time Zhao Yunlan took over, he finally sent in Chu Shuzhi to act as his proxy. Chu Shuzhi is pretty powerful, and since there don't seem to have been any truly big cases lately, he could detain any Dixingren culprits and deliver them to the Envoy without the Envoy having to meet anyone else.