MySQL Left Join returning unrelated records despite using IS NOT NULL

Tenderfoot :

The following query with left joins returns records that do not have a match on the final left join, i.e. the assessment column is null, though I have the query with IS NOT NULL .

How should the query be changed to get just the matching records?

Basically I want to return unit_outcome records GROUPED according to related assessments in the lookup table.

DB Fiddle for LEFT JOIN showing records with null for assessment

SELECT *
  FROM unit
  left JOIN unit_unit_outcome_lookup
    ON unit_unit_outcome_lookup.unit_fk = unit.unit_pk
  left JOIN unit_outcome
    ON unit_outcome.unit_outcome_pk = unit_unit_outcome_lookup.unit_outcome_fk
  left JOIN unit_outcome_assessment_lookup
    ON unit_outcome_assessment_lookup.unit_outcome_fk = unit_outcome.unit_outcome_pk IS NOT NULL
  left JOIN assessment
    ON assessment.assessment_pk = unit_outcome_assessment_lookup.assessment_fk IS NOT NULL
    AND unit.unit_pk ='1'

DB Fiddle for INNER JOIN with no null records - how to group unit_outcomes by assessment?

Akina :

SELECT MAX((unit.unit_pk)) AS unit_pk,
       GROUP_CONCAT(unit_outcome.unit_outcome) unit_outcomes,
       MAX(assessment.assessment) assessment,
       GROUP_CONCAT(unit_outcome.unit_outcome_pk) unit_outcome_pks, 
       assessment.assessment_pk
FROM unit
INNER JOIN unit_unit_outcome_lookup
    ON unit_unit_outcome_lookup.unit_fk = unit.unit_pk
INNER JOIN unit_outcome
    ON unit_outcome.unit_outcome_pk = unit_unit_outcome_lookup.unit_outcome_fk
INNER JOIN unit_outcome_assessment_lookup
    ON unit_outcome_assessment_lookup.unit_outcome_fk = unit_outcome.unit_outcome_pk
INNER JOIN assessment
    ON assessment.assessment_pk = unit_outcome_assessment_lookup.assessment_fk
    AND unit.unit_pk ='1'
GROUP BY assessment_pk;

fiddle

PS. Details can be found in the comments to the question.