It’s consistent and has been happening for a long time. I’m sure I mentioned it years ago. You tried a few changes, but they haven’t fixed my problem. I stopped mentioning it and save the chapters from Firefox every time.
However you format your text, the GA software doesn’t properly code the chapter as correct form of text. This prevents Safari from allowing Reader View mode. Firefox is less strict about observing standards and does allow Reader View mode. Reader View mode switches the background to white and the text to black (no matter what the site displays) and gets rid of extraneous junk like ads, navigation bars, sidebars, and other distractions. (Safari’s Reader View mode can optionally reverse the usual format to white text on a black background and allows you to select the font used [including making the font size larger or smaller], among other choices.) There have been one or two other authors whose text also exhibits the exactly same problem, but it’s been quite awhile since I’ve read any of their stories. ;–)