This is a very interesting question, because it is much easier and common to speak of how fiction becomes truth (apparent truth). If we look at it the other way around, it is much harder.
In chapter 26 we have an example of truth becoming fiction. Miss Gates, Jem's teacher, speaks about the rise of Hitler and his evil actions. From this we can say that Miss Gates is able to see clearly that Hitler is an evil leader. But she is not able to see what is going on in her own town.
The residents of Maycomb persecute blacks, and she does not see it. More pointedly, in Tom Robinson, there has been the death of an innocent black man. And for the most part the whites of the town did nothing about it. They do not even see a problem.
Jem is shocked at the blind spot. From this we can say that this an example of truth becoming fiction. Maycomb in truth is a racist town, but the residents of Maycomb have created a town where truth reigns and all is well, which is obviously a fiction.