It's worth noting that the film was already understood to be problematic back in 1946, even by reviewers who liked it:
Song of the South (Walt Disney-RKO Radio) makes movie actors out of ol' Uncle Remus, Brer Rabbit, Brer Fox and de creeturs an' crawlin' things. Adapted with freehanded skill from the famed...
content.time.com
Theirs.
www.nytimes.com
Quoted from the first link:
Ideologically, the picture is certain to land its maker in hot water. Tattered ol' Uncle Remus, who cheerfully "knew his place" in the easygoing world of late 19th Century Georgia (Author Harris, in accepted Southern fashion, always omitted the capital from the word "Negro"), is a character bound to enrage all educated Negroes, and a number of damyankees.
And from the second:
The National Association for the Advancement of Colored People expressed regret yesterday over Walt Disney's new production, "Song of the South," on the ground that it is helping to perpetuate the impression of "an idyllic master-slave relationship" in the South. Walter White, executive secretary, in telegrams to newspapers, stated the association recognized the artistic merit of the picture, but added, "It regrets, however, that in an effort neither to offend audiences in the North or South, the production helps to perpetuate a dangerously glorified picture of slavery. Making use of the beautiful Uncle Remus folklore, 'Song of the South' unfortunately gives the impression of an idyllic master-slave relationship which is a distortion of the facts."