Sometimes the article precedes the noun and not the adjective

You're conflating two different constructions. The second is required when using one noun to describe another, without converting the first to an adjective. The Irishism He's a jewel of a boy is the earliest I have found, but it's not unusual: in Walter Scott's Ivanhoe one character calls another "Dog of a Jew!"

The first is specific to words like that or so, used to qualify an adjective. ?A so big deal would never be used by a native speaker, in any context. As far as I can see, this is purely idiomatic; Shakespeare's So fair and foul a day I have not seen (Macbeth, I, 4) could be rephrased as I have never seen a day so good and bad, but not as *I have never seen a so good and bad day.