Ok, great. Yeah, the last point from @lcomm would explain the behaviour I was seeing. It was increasing the dimension of the simplex that seemed to enforce the smoothing, rather than changing the concentration parameter. I had \alpha_i = 1 for all i=1,...k in my example.
I’m not sure whether it’s desirable. Maybe it is? But it definitely wasn’t intuitively expected, at least not my me. I expected that increasing the degrees of freedom for the spline basis would increase flexibility, but that prior distribution had other ideas.