the term ‘hitherto unseen’ doesn’t need a hyphen, but it still triggers the hyphenation problem warning.
Correct, but not usual: This hitherto-unseen defect causes many problems.
Correct: This hitherto unseen defect causes many problems.
Correct: This defect was hitherto unseen, and it causes many problems.
NOW corpus (http://corpus.byu.edu/now/): hitherto unseen + noun = 96, hitherto-unseen + noun = 6, hitherto-unseen = 8.