Do I have to leave immediately after my immigrant visa comes down?

Legal analysis: you can't go abroad after getting a visa. After the visa takes effect, you must go abroad and leave the country within the validity period of the visa. For example, the visa takes effect from June 1 and is valid for 1 month, so you can leave on any day from June 1 to July 1.

Legal basis: Article 9 of the Law of People's Republic of China (PRC) on Exit and Entry Administration. China citizens shall apply for passports or other travel documents when leaving or entering the country. China citizens who want to go to other countries or regions also need to obtain visas or other entry permits. However, unless the China government has signed visa exemption agreements with other governments or the Ministry of Public Security and the Ministry of Foreign Affairs provide otherwise. China citizens who leave or enter the country as seafarers and work on foreign ships shall apply for a seaman's certificate according to law.