The following query gives zero results on WDQS, but QLever and Virtuoso both return hits, and correctly so I think:
PREFIX wdt: <http://www.wikidata.org/prop/direct/> SELECT ?person ?date_of_birth WHERE { ?person wdt:P569 ?date_of_birth . FILTER ISBLANK(?date_of_birth) } LIMIT 10
Try it on WDQS: https://w.wiki/8Szp
Try it on QLever: https://qlever.cs.uni-freiburg.de/wikidata/2nzFYg
Try it on Virtuoso: https://wikidata.demo.openlinksw.com/sparql
PS: Not sure whether this is the right page for reporting this. If there is a better place, please let me know.