Surprised by my own overlook

During a performance evaluation, one peer provided no-data-to-evaluate feedback to one of my reports. They surprised me. What did I overlook?

Let’s name the peer — Terra and my report — Ken. Ken is a young engineer. I prepared an individual project for Ken, for his growth. I also assigned several peers to him for help and performance evaluation. Terra was one of the peers. The project required building a data processing infrastructure and she was the most qualified person on the team in that area.