morphToMany relationship

I have 2 models in my app Category and SubCategory. A sub category can have many parents with it being either a Category or another SubCategory. So when trying to get the parents of a SubCategory i have set up the relationship as followed in the SubCategory:

public function parents(): morphToMany
    {
        return $this->morphToMany(
            Category::class,
            'parent',
            'sub_category_parent',
            'sub_category_id',
            'parent_id'
        );
    }

So the relationship is store in the 'sub_category_parent' table with the following structure:

sub_category_id
parent_id
parent_type

but when i call this relationship i do not get the parents of the given SubCategory

//parents is returned as an empty collection							
$sub = SubCategory::with('parents')->find(1);

I have made sure that the DB has the correct values. the weird thing is if i dd() the following in my parent function

dd($this->morphToMany(
            Category::class,
            'parent',
            'sub_category_parent',
            'sub_category_id',
            'parent_id'
        )->get());

i can actually see the parents.

So any help would be appreciated.