Using the group_concat function with the separator indicating an empty string ("") leads incorrectly to the default use of a space character as separator (" ").

Example code:

PREFIX : <http://example.org/>

SELECT ?subject (GROUP_CONCAT(?object; separator="") AS ?concatenatedObjects)
WHERE {
  VALUES (?subject ?object) {
    (:subject1 "a")
    (:subject1 "b")
    (:subject1 "c")
  }
}
GROUP BY ?subject

Expected outcome for concatenatedObjects:

abc

Instead we get:

a b c