Why will xAct/xCoba not contract these basis elements?

Question

Consider the following minimal working example:

<< xAct`xTras`
DefManifold[spacetime, 4, {α, β, γ, μ, κ, ν, ξ, ρ, ζ, τ}]
DefMetric[{3, 1, 0}, Met[-μ, -ν], CD, {";", "∇"}, 
 PrintAs -> "g"]
(* 1 *)
DefTensor[u[-μ], spacetime]
DefChart[coords, spacetime, {0, 1, 2, 3}, {t[], 
  r[], Θ[], ϕ[]}]
(* 2 *)
expr = u[α] PD[-α][u[-β]]
basisexpr = ToBasis[coords][%]
(* 3 *)
TraceBasisDummy[basisexpr]
ContractBasis[ComponentArray[%]]

In (1) I have defined a tensor and a chart. In (2) I define a single term of some expression:

$$ \text{expr}_\beta = u^\alpha \partial_\alpha u_\beta $$

Then ask to give the result in a basis (3). However, it will not replace the free indices with the basis indices:

ContractBasis does not work. I assume there is a function to tell xAct that the basis vectors can move through the partial derivatives (perhaps the mixed indices are confusing it) but I can't find it in the manual now.

Thanks