I think he slowed because he was in poor position to set a PB. Getting the win became the important objective. I don't think he expected to get passed by Jakob and I don't think Jakob made a legal pass tbh. It was a good time to test his kick and control a race.
Looked like Tim intentionally slowed down after the pacer dropped, costing the whole field a shot at a fast time. Maybe wanted to test out his kick in preparation for Worlds, but it was strange to see after he was passing the pacer early.