To be honest I have been struggling to understand the relationship between the PRC and DPRK. I know geopolitics is difficult and that there has been a lot of mutual aid and cooperation between the two states, but I find it hard to explain stuff like this (from Wikipedia):
China condemned the 2006 North Korean nuclear test, as well as the subsequent nuclear tests in 2009, 2013, January 2016, September 2016 and 2017. China abstained during a 2017 United Nations Security Council vote about sanctions on North Korea, leading it to be approved.
Well... People fuck around and seems to have been doing so for a while...