Why is an article inserted before a proper noun that has been qualified by an adjective?

Proper nouns in English are not normally qualified by adjectives; the adjective would be taken to be part of the proper noun (This is Lucky Phil).

Some authors do qualify proper nouns with adjectives, although as this discussion notes (Adjective with proper noun), it is stylistically quite marked (“Stylistically, attributively modifying a proper noun isn’t something people do in normal conversation. It strikes me as newspaper-ese”.)

When that does happen, the proper noun is considered to be acting more like a common noun: it’s as if the adjective is being used to narrow down which of the avatars of the person is being considered. (A bewildered Elliot, as opposed to a contented Elliot; The Amazing Mr Fox, as opposed to The Humdrum Mr Fox.) Hence the use of the article.