On https://www.mediawiki.org/wiki/Special:PrefixIndex/Community_Engagement_%28Product%29/Collaboration_process the title [[Community Engagement (Product)/Collaboration process/Draft]] has unnatural whitespace before the parenthesis.
|mediawiki/core||master||+21 -13||Don't apply CSS columns if less than 3 results were found on AllPages & PrefixIndex|
So I think that the root issue here is that the browser is trying to render it as it would appear for 3 columns even if we've just one entry in the resultset. I tried playing around with the CSS but it doesn't appear correctly on all screens so I think the best thing to do here would be to just show a normal list if there's less than 3 entries. CategoryViewer also does this.