No, but I did encounter this (see attached screen capture) after I displayed the Reading Navigation toolbar and the Preview Pane then went back to the page. After the Preview Pane closed, the top paragraph was displaying the font much thinner than it had before. It had looked like the bottom paragraphs. This is in an eBook I modified to show all the text in bold. I think the top paragraph somehow lost its bold setting. I read about 3 hours in this book and this anomaly only occurred once. I did see this fairly often in the PW1s though.

EDIT: Depending on your monitor, you might need to zoom the attached image to see what happened. To zoom it, click/tap on the attached image below, after it displays in the pop up window click/tap on it again and it will display in its own browser tab or window. Then click on it again from inside its own browser tab or window and it will zoom to actual size.

......