When changing a Word document to display a black background (changing the page color), not all the black text changes to white. It happens on different sections of the document and even part way through a sentence ! (all black text with a white background, but when changed, black and white text with a black background). I've copied into a new document, but the issue occurs in the new document also. When highlighted, the text that is displayed in white and black are the same, same font, size, format, etc. I haven't changed the default (black text, white background). Do you have any idea of a probable cause?
Check the actual color option for the text. Black text should read "black" while white text should read "automatic". Automatic text color adapts itself to your background, while Black will always remain black.
Try resetting font style to default or applying Automatic color for all the "problematic" text.