I’m not entirely sure what’s causing the initial font changes, but… I know in Word, you have the option to select a format for different sections of a document, like “Heading” or “Title” or “Normal” and those affect how the text appears. You can apparently set different fonts for some format options, I believe. If that’s what happened, you can fix that by selecting just one format for the entire document. I usually go with “No Spacing” myself.
There’s a similar format option in my email interface as well—I use Thunderbird to see all my email addresses in one place. (Don’t ask. It’s complicated.) I can choose between things like “Body Text” or “Paragraph” or “Heading” which then formats my text. It’s useful if I’ve copy-pasted something into the email that doesn’t match the format of the email client.
Other than that, you can try selecting all of your chapter in the Rich Text Editor, and then setting a single font and size for the entire document.