Steps to reproduce:
- Edit a page in "Source editing" mode
- Copy text that contains paragraph break and some formatting (e.g. content of https://www.mediawiki.org/wiki/User:Sebastian_Berlin_(WMSE)/Newline ).
- Paste into editor.
- Select "Convert to wikitext" in the popup.
Pasted text contains only one newline, which is rendered as a space.
Pasted text contains two newlines.
Using Firefox 60.0.1