Debugging requires an extremely thorough understanding of the project, so much that you can not only find each responsibility in the code, but anticipate their emergent properties, what mistakes might have been made, and how their interactions might go wrong. Debugging a system successfully is the culmination of successful on-boarding, not a starting point.

Doing the actual code changes to fix a bug which has already been analyzed and localized to one or a few sites in the codebase is a bit more reasonable.