I think you need to this,
https://github.com/bagisto/bagisto/blob/710e1515fdfad46e3086dfecab61a1fd8cd5503b/packages/Webkul/Category/src/Models/Category.php#L52
This is already appended with attributes here,
https://github.com/bagisto/bagisto/blob/710e1515fdfad46e3086dfecab61a1fd8cd5503b/packages/Webkul/Category/src/Models/Category.php#L64