While doing metrics, @mpopov found a bug using this URL: https://en.wikipedia.org/w/index.php?search=locuteurs+natifs&title=Special:Search&fulltext=1
Note: it doesn't happen with this URL: https://en.wikipedia.org/w/index.php?search=locuteurs&title=Special:Search&profile=default&fulltext=1
Here's the screenshot before a different minor UI fix (T146655):
And, after the minor UI fix (T146655):