Could you tell me what theme you are using and what version of Moodle you are using?
When testing in Moodle 3.5 with the More theme which seems similar in style to what you're using, I found that it would show sub categories when I clicked into a main category. I'm not aware of a setting that would cause this.
When testing in Moodle 3.5 with the More theme which seems similar in style to what you're using, I found that it would show sub categories when I clicked into a main category. I'm not aware of a setting that would cause this.