When paragraph bottom margin was switched to padding, the rule which removes bottom margin from the last child in blockquote was not updated to also remove the padding.
This was "fixed" on en.wiki's Template:Quote_box by adding a <p style="padding-bottom: 0">: https://en.wikipedia.org/w/index.php?title=Template%3AQuote_box&diff=1209709036&oldid=1205466757 however this has the side effect of always rendering a paragraph even if there is no author.