I noticed that Customizr appears to place the container for the left tc-sidebar underneath article-container on small media screens.
My PHP coding skills are slightly rusty but I did look into the index.php and class-content-sidebar.php files. It would seem the left sidebar is supposed to be rendered before article-container but it doesn’t seem to be the case here.
The behavior is the same regardless if the layout is set to use both sidebars or only the left sidebar. I suspect this to be a feature since on smaller screens you might want the widget field to be less prioritized than the widgets.
But I have been wrong before and personally I’d like the left sidebar to be placed before article-container.