Could be that the Character art and the background image were created by two separate artists. While it’s uncommon for a commissioned artist to be more than one person, there are collaborative art shops/kofis/drives so perhaps they require that they both are credited.
I think any situation like this would be pre-arranged and agreed upon before the artist created the outline/sketch. So, I’d say yes, if it’s already been paid for and permission has been given, an artist would be okay with someone filling in the color/shading etc.