What does the amber label next to a chapter on the book's page mean?

Each finished chapter shows a small label with the engine and model that wrote it. From version 1.9.21 the label turns amber when the chapter was not written by your book's engine or by the model you chose: a fallback wrote it because yours was busy, rate limited or refused. Hover the label to read the details, for example which model you wanted and which one wrote the chapter. Nothing is wrong with the chapter; it is only written by another model. To write it with yours, regenerate that chapter (the old text stays in Versions). To stop fallbacks altogether, choose "Stay on my engine" under "If my engine fails" in Settings.