WARNis the appropriate severity because the operation technically succeeded, yet the result is abnormal enough to require attention and could produce undesirable side effects. A warning signals a recoverable or unexpected condition without incorrectly classifying the operation as an outright failure.
Salesforce logging guidance identifies custom WARN logs as appropriate for warnings in application areas such as integrations and payment processing that can impact users. WARN messages are also available under normal production logging configurations, making them suitable for operational conditions that administrators should be able to investigate.
DEBUG is intended for detailed development/troubleshooting information and is disabled for persistent production logging by default. It would therefore understate a potentially consequential condition. ERROR should represent an operation or state that actually failed or cannot be processed successfully; the question explicitly states that the operation succeeded.
Using the correct severity improves alerting, log filtering, and incident analysis while preventing routine recoverable situations from being mistaken for application failures.
Study Guide reference:Application Development -dw.system.Logger, WARN/DEBUG/ERROR severity, Service Framework troubleshooting, and operational logging practices.