Arxiv Empty Bibliographic data
I eventually found some preprints that happened to show this bibliographic data section, e.g. 1805.02262. Clicking the What is Bibex? link, I note that the current version of the arXiv Bibliographic Explorer is 0.1, from September this year. In addition, they refer to this functionality as a project. Hence, I think you're looking at a somewhat experimental feature, still in development. Most likely then, this feature simply hasn't been able to extract the data it needs, for whatever reason. You could use the "report data issues" link to send them a feedback email to help the development forward.