If you are in a section (have selected the section on the right so that it is the only thing visible), and you click on the “Add Case” button (on the right of the top strip), you are taken into the full “Add Test Case” view, where you create your test and click “Add Test Case” at the bottom to save.
Unfortunately, this process creates the Case in the root section, not the section that you were “in” when you started the process.
I am aware that you can select the section once you are in the full “Add Test Case” view, but this should default to the section that you started in, not the root.
I am also aware that you can click the “Add Test Case” link at the bottom of the section that you are in to create a Case directly in that section. But, firstly, that is a two step process (create, then edit to provide detail), secondly, the “Add Case” button (which is not behaving as expected) is more visible.