I'm no expert in this area, but the Post had an article on Kim's visit to China which caused me to think.
Based on our experience with sanctions against various countries: Iran, North Korea, Russia, etc., I draw this lesson: to some extent imposing sanctions is a moral cascade--there's a triggering event which gets leaders of countries/the ruling class upset and determined that "something must be done". The answer is imposing sanctions. Sometimes the sanctions are more for show, rather like arresting a few prostitutes used to be back in the middle of the last century, or cracking down on gambling in a gin joint in Casablanca. But sometimes the outrage is enough to support strong sanctions, sanctions that hurt.
This seems to have been the result with Iran before the nuclear deal and North Korea after the tests of long range missiles and the hydrogen bomb. It's hard, however, to sustain outrage. It's particularly hard when the leaders who imposed the sanctions, Xi and Trump, are making nice with the leader of the sanctioned company. The sanctions may be in effect still, but the bureaucrats who have the job of enforcing them aren't going to have their hearts in it. They know there's not going to be calls from the leader's office asking them "what did you do today to make life hard for North Korea."
The analysis of the Singapore summit has been that Trump didn't give Kim anything which couldn't be reversed in the future, except the first meeting with the US president. But that analysis will be wrong if the sanctions are slowly eroding because of the change of attitude at the top,