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The integration of artificial intelligence into materials science has accelerated discovery through iterative workflows that cycle through data acquisition, model refinement, prediction, explanation, and hypothesis-driven experimentation. While explainable artificial intelligence (XAI) methods enhance trust and scientific insight by elucidating model decisions, these explanations are not static. This manuscript introduces the novel concept of explainability drift: the systematic degradation, inconsistency, or divergence in the fidelity, stability, and relevance of XAI-generated explanations across successive iterations of materials AI workflows. Distinct from prediction-focused concept drift, explainability drift arises from evolving data distributions, model updates, feature space expansions, and domain shifts inherent to materials exploration. Through a purely conceptual failure analysis, we delineate the mechanisms underlying explainability drift, including temporal instability in feature attributions, erosion of surrogate model alignment, and semantic misalignment between explanations and emerging material knowledge. Drawing on recent peer-reviewed advances in XAI applications to property prediction, microstructure analysis, and generative design, we synthesize theoretical foundations to highlight why drift undermines iterative efficacy. The proposed conceptual framework organizes explainability drift into multidimensional layers—attributional, structural, and epistemic—offering a structured lens for analyzing failure modes without empirical validation. This framework emphasizes risks such as misguided hypothesis generation, diminished trust in AI-assisted insights, and inefficient navigation of vast materials design spaces. By conceptualizing explainability drift as an intrinsic challenge, the work advocates for theoretical advancements in sustained explainability to support robust, interpretable AI-driven materials innovation.