Since SC update of march 2019, an option allows you to load categories (in the left hanside column) dynamically, which is very useful if your shops has a large number of categories as it would improve the display performance.
Ineed, when the option is enabled, only parent categories are loaded.
Sub-categories will only be loaded when you expand their parent category.
To improve display performances when working with your categories, enable this option in Store Commander > Tools > Settings > Catalog > Category > Enable dynamic loading for category tree
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