how to reproduce

• in the configuration of the site admin check the boxes
• from the back office create a category with a newsletter.
• create a subcategory with a newsletter
• create in a page the newsletter subscription service to the parent category
• from the front office subscribe to the newsletter
• from the back office send the newsletter of the subcategory

The user is subscribed to the parent newsletter and the subcategory.
However it would be good to make a yellow banner in the back office to say that users of the subcategories will receive the newsletter

• Modify the newsletter subscription service in the back office.
  Change parent category to subcategory
• from the front office subscribe to the newsletter
• In the back office send the parent category newsletter

The user is subscribed to the newsletter of the subcategory and the parent newsletter.
However it would be good to make a yellow banner in the back office to say that users of the parent categories will also receive it.