Missing data in custom SQL report

[VOOM108]

I wrote an SQL query that works on all Moodle platforms I support, except one. The strange thing is, that out of 1800 users that are enrolled in a course, only 1200 show up in the report. There is a similar effect with all courses using this report. There is no obvious reason for that discrepancy.

Any idea what may be going on here? What factors could be at play here?

SELECT 
 c.shortname as "Kurs",
 u.lastname as "Nachname", 
 u.firstname as "Vorname", 
 u.institution as "Firma",
 u.department as "Abteilung",
 FROM_UNIXTIME(ue.timecreated) as "Eingeschrieben",
   (
    CASE 
      WHEN cmp.timecompleted IS NULL THEN 'ungueltig' 
      ELSE 'gueltig' 
    END
  ) as "Zertifikat",
 FROM_UNIXTIME(cmp.timecompleted) as "Aktueller Abschluss",
  (
    SELECT 
      GROUP_CONCAT(FROM_UNIXTIME(cc_hist.timecompleted) SEPARATOR ', ') 
    FROM 
      prefix_local_recompletion_cc cc_hist 
    WHERE 
      cc_hist.course = cmp.course 
      AND cc_hist.userid = cmp.userid 
  ) as "Archivierte Abschluesse, Komma-sep."
FROM 
  prefix_user u
  LEFT JOIN prefix_user_enrolments ue ON u.id = ue.userid
  LEFT JOIN prefix_course_completions cmp ON cmp.userid = u.id 
  LEFT JOIN prefix_course c ON c.id = cmp.course
WHERE 
  cmp.course = %%COURSEID%%
  AND u.suspended = 0
GROUP BY 
  u.id, 
  cmp.course

It's not the u.suspended = 0, this filters out only very few as intended.