Steps to reproduce:
- Edit a page.
- Add a <blockquote> tag with two paragraphs in it, with empty line between.
The HTML contains one paragraph.
The HTML contains two paragraph.
I did some tests over at https://www.mediawiki.org/wiki/User:Sebastian_Berlin_(WMSE)/T309778 with different ways of making two paragraphs. The way that I got to work was to explicitly add <p> tags.