I agree with KevinH. Check Preview's Inspector, especially the Sources tab.
I can cause this effect, e.g. by adding a code:
Code:
html {
-webkit-transform: scale(1, -1);
}
to the custom_preview_style.css file in Sigil configuration folder.
So the culprit is the
scale(1,-1) entry – search the ebook code and the Sigil configuration folder to see if it appears somewhere.