I am trying to use the single.html custom layout for rendering the single pages I have in the content\concerts section folder. (concert1.md, concert2.md, etc…).
I suggest placing the templates under layouts/concerts and then within said directory create a list.html for the section list and a single.html for the section’s single pages.
I think that with your current setup Hugo does not pickup the single template under the concerts directory because it encounters the concerts.html list template at the same level and processes it first, while ignoring the directory with the same name.