Beyond Localization: A Comprehensive Diagnosis of Perspective-Conditioned Spatial Reasoning in MLLMs from Omnidirectional Images

Multimodal Large Language Models (MLLMs) show strong visual perception, yet remain limited in reasoning about space under changing viewpoints. We study this challenge as Perspective-Conditioned Spatial Reasoning (PCSR) in 360 degree omnidirectional images, where broad scene coverage reduces ambiguity from partial observations without eliminating the need for viewpoint-dependent inference. To assess this capability, we introduce PCSR-Bench, a diagnostic benchmark of 84,373 QA pairs from 2,600 omnidirectional images across 26 indoor environments, organized into eight tasks under three cognitive groups--- Perception, Spatial, and advanced PCSR. We evaluate 14 representative MLLMs and observe a substantial perception--reasoning gap: accuracy reaches 57.59% on Limited Field-of-View Reasoning (T7) but drops to 13.49%, 7.13%, and 0.64% on Relative Direction (T2), Egocentric Rotation (T4), and open-ended Compositional Directional Chains (T3), respectively. To probe the plasticity of this gap, we conduct an RL-based diagnostic study on a 7B-scale model. Reward shaping improves a matched 7B baseline from 31.10% to 60.06% under a controlled setting, suggesting that PCSR exhibits partial plasticity rather than being fully immutable. Still, these gains are task-selective, sensitive to reward design, and partially dependent on the evaluation protocol. These results position PCSR as a key bottleneck in current MLLMs and highlight meaningful yet bounded room for recovery under targeted optimization. Details and access are available at https://github.com/Caleb-ychen/PCSR-Benchmark.