[paperback]

AFAIK, this didn't happen before Calibre 6.12.

1. Open the attached ebook (Demo) in the ebook editor and viewer.
2. Look at the style file in the editor and notice that the blockquote top and bottom margin is clearly set to 1em.
3. Adjust the viewer window so that the document occupies two pages. Notice that the blockquote bottom margin is being ignored.
4. Remove the surrounding div tags and the blockquote bottom margin reappears.

The trigger factors are (1) the tag with a bottom margin is enclosed wthin div tags and (2) the paragraph spans two pages (or more?) of the ereader.