I already wrote a comment about this but I'll rewrite it here: 

When SeonMi was under the impression that she hurt Son Oh Gong because she called his name. Nobody bothered to correct her by telling her that he got hurt because he disobeyed her orders thus the GGG punished him for it. They used that plot hole to make unnecessary drama by having Seon Mi go into Asanyeo's coffin and lock herself there. When in reality, all this stupidity could have been avoided.

The drama further shot itself in the foot by having Ma Wang bring the old version of GGG to test it out. To prove that when Ma Wang disobey orders, he gets pain. That's why the characters didn't correct her because either the writer forgot their own rules or they ignored it to create drama.