Does quantum back-reaction forbid the formation of time machines?

For a spacetime that develops a compactly generated Cauchy horizon (chronology horizon) from regular initial data, the renormalized $\langle T_{ab}\rangle$ of a linear quantum field is not a well-defined distribution at certain base points of the horizon (Kay, Radzikowski, Wald 1997). Determine whether semiclassical or quantum gravitational dynamics generically prevents closed timelike curves from forming, given that the breakdown occurs within a Planck distance of the horizon where the semiclassical approximation itself fails. An answer is a theorem in a controlled approximation, or a model spacetime in which closed timelike curves form with bounded $\langle T_{ab}\rangle$ and curvature below the Planck scale.

A proof that back-reaction excludes compactly generated chronology horizons under stated assumptions, or an explicit counterexample with sub-Planckian curvature throughout.