[Certain]

I understand the connotative differences. But spelling in formal settings tends to go with denotative word. A fair argument would be whether closed captioning should be constrained by formal writing styles, but it generally is. Now if you saw "shoulda" in a closed caption, then you'd have a better argument.

Also, I'm not black and don't particularly care whether black people use it. But there still are a lot of black people who view the two words as one in the same.