Isn't that expected because of the automatic re-balancing to minimize risk? I don't think anyone goes with target date funds thinking they are going to maximize returns because it's a tool to buy and forget.
But buying VTI is also a tool to buy and forget and it maximizes returns compared to target date funds. They're lower due to fees as well as investing too much into bonds when young when it's not necessary, one should invest in bonds when closer to retirement instead.
