Large Language Models (LLMs) have become tools for software development and maintenance, demonstrating impressive capabilities in automating tasks such as code generation and refactoring. However, it remains empirically unclear whether the rapid scaling of these capabilities is accompanied by proportional improvements in security, or whether a discrepancy, defined as the Security–Functionality Gap, persists across model generations. To address this question, we conducted an evolutionary study analyzing the trajectory of 12 state-of-the-art models across the GPT, Llama, and Claude Sonnet lineages. By leveraging an outcome-driven benchmark based on dynamic execution, we quantified the divergence between the models’ ability to generate functionally correct code versus secure code over four sequential versions per lineage. Results reveal that, while functional correctness has effectively plateaued (> 90%) across all families, functional-security correctness has evolved more slowly. Specifically, while proprietary models demonstrate marginal improvements, open-weight models exhibit stagnation, and the most advanced models occasionally introduce new regressions due to increased code complexity. These findings suggest that security alignment is not an intrinsic byproduct of model scaling, highlighting the urgent need for external guardrails in AI-assisted maintenance pipelines.