We're experiencing an issue with CE 2.1.9's mobile menu. When you click on the parent category, all the 2nd and 3rd level subcategories are also opened. Clicking on the parent should only open the 2nd level categories of this parent. As I've heard, it's a bug of Magento's, so my question is - when will it be fixed?
Are you using a custom theme or the error is with Luma theme?