This study evaluates whether an induced HRIS-consistent reasoning regime remains stable under controlled perturbation. While prior work has demonstrated that initialization signals can influence early trajectory selection in language model inference, it remains unclear whether such regimes exhibit persistence or collapse under variation in input conditions. A fully specified and reproducible protocol was developed consisting of ten independent trials. Each trial was conducted in a fresh session using an identical base task and constraint initialization, with a single perturbation applied per condition. Perturbations included stylistic variation, task transformation, epistemic ambiguity, contextual noise, and competing mode signals. Results demonstrate consistent preservation of core reasoning structure across all perturbation classes. Observed variation was confined to surface-level expression, including tone, format, and representation, while underlying assumptions, constraint application, and reasoning pathways remained invariant. Under competing mode conditions, alternate instructions did not displace the active reasoning regime but were incorporated in a subordinate manner, preserving primary structural coherence. Stability is defined behaviorally as persistence of reasoning structure under controlled perturbation, rather than through direct observation of internal model states. These findings support the interpretation of HRIS-consistent behavior as a stable inference region, rather than a transient stylistic effect. The results establish a foundational condition for subsequent validation studies examining basin selection, activation thresholds, and trajectory persistence in language model inference. More broadly, these results bear on a foundational question in the philosophy of artificial systems: under what conditions can consistent reasoning be attributed to a system whose internal states are not directly observable, but whose behavior exhibits structured invariance across perturbation.