Autonomous coding agents (e.g., OpenAI Codex, Devin, GitHub Copilot) are increasingly used to generate fix-related pull requests (PRs) in real-world software repositories. However, their practical effectiveness depends on whether project maintainers accept and merge these contributions. In this paper, we present an empirical study of AI agent–involved fix-related PRs, examining both their integration outcomes, latency, and the factors that hinder successful merging. We first analyze 8,106 fix-related PRs authored by five widely used AI coding agents from the AIDEV-POP dataset to quantify the proportions of PRs that are merged, closed without merging, or remain open. We then conduct a manual analysis of a statistically significant sample of 326 closed but unmerged PRs, spending approximately 100 person-hours to construct a structured catalog of 12 failure reasons. Our results indicate that test case failures and prior resolution of the same issues by other PRs are the most common causes of non-integration, whereas build or deployment failures are comparatively rare. Overall, our findings expose key limitations of current AI coding agents in real-world settings and highlight directions for their further improvement and for more effective human-AI collaboration in software maintenance.