Hi Vincent,
We think the problem is caused by widget setting.
Please go to admin->appearance->widgets->shop sidebar area.
In there, please open the "Product Categories" widget and set the "Maximum Depth" value by 2 or empty.
https://prnt.sc/110t1gl
Maybe, current value will be 1.
Please try. And if you have any other problems, please let us know.
Best Regards, C-Themes
If you have the other questions, please let us know again without hesitation.
Best Regards, C-Themes