Compare [1] to [2]. [1] is in German, and German has the label "". Dutch [2] has no label:

German: "Wesel: Unterschied zwischen den Versionen"
Dutch: ": verschil tussen versies"

It looks like "title" is empty when there's no label in the user language for the data. It's not an i18n issue in MediaWiki, as when a title in "nl" is known, the message can be "Rondwormen: verschil tussen versies" per [3]

[1] https://www.wikidata.org/w/index.php?title=Q4011&diff=269480&oldid=265671&uselang=de
[2] https://www.wikidata.org/w/index.php?title=Q4011&diff=269480&oldid=265671&uselang=nl
[3] https://www.wikidata.org/w/index.php?title=Q5185&diff=272224&oldid=272222&uselang=nl

Version: master
Severity: normal
URL: https://www.wikidata.org/w/index.php?title=Q4011&diff=269480&oldid=265671&uselang=nl