So, no matter what I do, Wattpad is introducing formatting errors. Here's the reason and solution below. It's a lot of work. It'll take some time. So apologies until I can do this for both books.
From the reply to my help request:
"The issue you are describing is related to a system update that causes the formatting of a Word document to be no longer compatible with the Wattpad writing page.
"We recommend using Google Docs to create drafts and copy them over to Wattpad through the website (www.wattpad.com). This will ensure that most of your text formatting remains, such as bold and italics. Unfortunately, though, text alignment might not be saved when pasting the content on Wattpad. I really apologize for the inconvenience caused by this. This is something we're hoping to resolve in the future.
"If you have already written your story on Word, you can still copy and paste it to Google Docs, ensuring the formatting is correct, and then copy and paste the content from Google Docs to Wattpad."