Exact match of variable string in SPARQL Wikidata Query Service

Question

Exact match of variable string in SPARQL Wikidata Query Service at https://query.wikidata.org does not give the the results I expected.

I was expecting I could do:

SELECT * {
  hint:Query hint:optimizer "None" .
  { SELECT DISTINCT (xsd:string(?author_name_) AS ?author_name) { wd:Q5565155 skos:altLabel ?author_name_ . } }
  ?work wdt:P2093 ?author_name .
} 

But I get no returned results from the Wikidata Query Service:

However, if I use the "=" comparison, I can match the strings:

SELECT * {
  hint:Query hint:optimizer "None" .
  { SELECT DISTINCT (xsd:string(?author_name_) AS ?author_name) { wd:Q5565155 skos:altLabel ?author_name_ . } }
  ?work wdt:P50 wd:Q5565155 .
  ?work wdt:P2093 ?author_name__ .
  FILTER (?author_name = ?author_name__)  
} 

With the current data in Wikidata, I get five rows returned in this query.

Another way to get this data is by using a BIND:

SELECT * {
  BIND("Knudsen GM" AS ?author_name)
  ?work wdt:P2093 ?author_name .
} 

I suppose there might be something wrong with the casting as this does not return anything:

SELECT * {
  BIND(xsd:string("Knudsen GM") AS ?author_name)
  ?work wdt:P2093 ?author_name .
} 

Combinations with xsd:string changed to STR or no conversion at all in the original query do neither yield result rows.