Reported here: https://www.mediawiki.org/wiki/Topic:Vws56wply7n661h0
Edit made in the background: https://en.wikipedia.org/wiki/Special:Diff/986155840
The users edit that run into a conflict: https://en.wikipedia.org/wiki/Special:Diff/986157929
What the user had to fix because of the lost line breaks: https://en.wikipedia.org/wiki/Special:Diff/986158379
I played the edits back, and can reproduce the issue.