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Do Multimodal LLMs Understand Order? Measuring the Fragility of Multimodal Reasoning under Input Order Perturbations

Multimodal reasoning has progressed rapidly with large vision-language models (LVLMs), yet their robustness under input variations remains underexplored. This study investigates positional bias in LVLMs for multimodal multiple-choice questions. Our analysis shows that model predictions are sensitive to both choice and modality ordering. We conduct a large-scale evaluation on MMMU, CVQA, and MMBench using fourteen representative models. Further analysis examines how question properties, including difficulty, domain, and image type, affect robustness. We also assess whether text-based mitigation strategies transfer to the VQA setting and perform ablation studies on self-consistency and reasoning complexity. Overall, our findings provide the first comprehensive understanding of positional bias from a vision-language perspective, highlighting key challenges in achieving stable multimodal reasoning.