Subset-Conditioned Boundary Compensation for Missing-Modality Multimodal Classification

Missing modalities reduce observations to arbitrary subsets, challenging robust multimodal inference. Existing methods recover missing views or align cross-subset representations, implicitly equating feature completeness with decision accuracy—leaving subset-specific decision boundary variation structurally unaddressed. Empirical analysis reveals that misclassification patterns are highly subset-specific, stable across initializations, and concentrated near decision boundaries—a persistent boundary misalignment distinct from information attenuation, which we define as Subset-Conditioned Boundary Shift (SCBS). To address this, we propose Subset-Conditioned Boundary Compensation (SCBC): a modality-factorized violation memory accumulates historical records of subset-specific margin violations, and a subset-adaptive head converts them into targeted logit corrections at inference, with a direction-consistency loss enforcing alignment during training. We prove that these corrections, built entirely from forward-pass statistics without backpropagating through the correction path, satisfy the descent condition of the surrogate loss, to our knowledge the first such guarantee in incomplete multimodal learning. Across five benchmarks, SCBC achieves state-of-the-art-level performance, averaging a 3.4 Macro-F1 point gain under random missingness and peaking at 7.4 points on the IEMOCAP-4 {v, a} setting. Source code is available at https://anonymous.4open.science/r/SCBC_neurips-50B5.