Debugging activities typically include reproducing the failure, diagnosing the root cause, and fixing the cause of the failure. Adding new test cases, while important in the overall testing process, is not a direct part of debugging. Debugging focuses on understanding and correcting the issue that caused the failure, rather than expanding the test suite.
References:
ISTQB CTFL Syllabus V4.0, Section 5.5 on debugging activities, which lists reproducing the failure, diagnosing the root cause, and fixing the cause as core activities​​.
