To which category of cost of quality does "fixing a defect found during testing" belong?

Comprehensive and Detailed Explanation From Exact Extract of ISTQB Certified Tester Advanced Level – Test Manager v3.0 syllabus:

The syllabus differentiatescost of qualityinto:

Prevention(to avoid defects),

Appraisal(to evaluate products, e.g., reviews, testing),

Internal failure(defectsfound before release, including the cost to fix during testing), and

External failure(defects foundafter release).“Fixing a defect found during testing” is aninternal failure cost, because the failure is detected and corrected prior to delivery (CTAL-TM v3.0, Organizational Test Management – economics of testing and cost of quality categories).