Multimodal continual learning (MMCL) aims to acquire new knowledge from multimodal data while retaining previously learned knowledge. Existing MMCL methods primarily mitigate forgetting by aligning cross-modal representations or preserving feature-level semantic similarity. However, different tasks may rely on different modalities, and learning new tasks can alter how modalities contribute to predictions on previously learned tasks. It remains underexplored how modality contributions evolve across incremental stages and how such changes relate to forgetting in MMCL. We term such changes Modality Contribution Drift (MCD) and introduce an MCD score based on controlled modality-subset interventions. Our theoretical and empirical analyses show how contribution drift can lead to forgetting, while existing MMCL and conventional CL methods do not effectively mitigate MCD. To address this issue, we propose Continual Modality Contribution Drift Regularization (CMCDR) to preserve the modality contribution profiles of previously learned tasks in both replay-based and replay-free settings. In the replay-based setting, CMCDR estimates modality contributions from stored old samples and regularizes their drift relative to a frozen previous model. In the replay-free setting, CMCDR uses current-task samples as probes to match old-class contribution profiles between the current and frozen models without storing old exemplars. Across six benchmarks covering multimodal class-incremental learning and continual multimodal question answering, CMCDR substantially reduces modality contribution drift, improves average accuracy by 0.88-7.66 percentage points, and reduces average forgetting by 0.85-11.55 percentage points over the corresponding baselines.

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