PMP Practice: Value servant leadership

Thandiwe Ndlovu is leading an agile team at Prairie Wind Logistics developing a mobile app for a municipal government client. During the retrospective at the end of Sprint 4, two senior developers, Kenji and Dante, publicly argue about technical approach. Kenji accuses Dante of ignoring his architectural suggestions, while Dante claims Kenji has been undermining his decisions in side conversations with junior developers. The product owner mentions this tension has been visible for two sprints. Team velocity has dropped from 42 story points in Sprint 2 to 28 story points in Sprint 4. Other team members appear uncomfortable and avoid eye contact. The retrospective ends early with no actionable improvements identified. What should Thandiwe do to address this situation most effectively?