The WDQS is returning results that differ from what is in Wikidata. See T112397 for previous discussions.
Example:
Run the following SPARQL query:

select * where {
  wd:Q21990261 p:P361 ?s .
  ?s ps:P361 wd:Q24772701 .
  ?s prov:wasDerivedFrom ?ref .
  ?ref ?b ?c .
}

I get one row as a result:

If you look on Wikidata: https://www.wikidata.org/wiki/Q21990261#P361, this is missing the stated in on the reference.

If you run the same query with replacing the wd:Q21990261 with wd:Q21135100, you'll see the query works just fine.