When a build contains more than 20 commits, all_commit_details is truncated to the last 20 commits. The job UI page will display "Commit List" under the "Commit" tag if this occurs:
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The API v1 response will respond with all_commit_details_truncated set to true when this occurs.

If you have automations that rely on information involved in commit messages you can send a final commit before you build with that information, as the most recent commits will be listed in the response.