To reproduce the issue:
1. Download, say, "The Voyages of Dr. Dolittle" from
Standard ebooks.
2. Add a style rule to the Calibre Viewer – line-height: 140%.
3. Add a bookmark to a few random pages in the ebook, preferably in the middle of chapters (right click on the page and choose "Bookmarks; or press Ctrl+ B).
4. Go to another page, then try to return to a bookmarked page (press Ctrl+B and click on a bookmark title)
Actual result: You return to another page not bookmarked.
If you delete the style rule, the problem disappears.