A Correct Mask Hides the Source of Failure: Two Kinds of Observation Aliasing in Learned Routing

Learned routing policies act under a correct feasibility mask, yet can still rank feasible actions poorly. We show that such a quality failure has more than one source, and that a correct mask hides which one: the deficit depends on where the decision-relevant information that the decoder observation aliases resides. In controlled experiments with a construction policy implemented independently of any released system, we exhibit a dissociation. When the aliased quantity is a state that accumulates along the route, adding decoder memory reconstructs it and removes 95% of the deficit over fifty-two training seeds. Two comparisons place that recovery: a content-free channel of the same width removes nothing, while the same recurrent module fed a constant removes 48%, mostly because its state is zeroed at each depot visit. Driving that reset from a different instance's depot schedule, matched to about a percent in how deep the state is when it is read, removes nothing (to within 0.07): what the reset supplies is alignment with the instance's own tour, not re-anchoring at the right rate. About half the recovery comes from depth into the tour and half from the route's accumulated state. When the aliased quantity is a per-instance latent regime that leaves no signature in the observed stream, decoder memory cannot reconstruct it and does not help; only an explicit regime signal closes the deficit. The same added memory therefore repairs one failure and leaves the other essentially unchanged, and the separation holds across regime strengths in the latent case and across model widths in the reconstructible case. We then revisit a released multi-task routing model whose mixed-backhaul failure combines both ingredients, an unseen regime and an aliased feasibility state, and where, after fine-tuning on a pool that covers the regime, a one-bit regime signal raises the repair of its 6.2 pp zero-shot deficit from 0.47 to 0.88. In practice, therefore, the source must be diagnosed before a fix is chosen. The contribution is a mechanism-level dissociation: two hidden sources of failure that a correct mask renders indistinguishable, and that demand different fixes.