Tectonic says warnings were issued but I cannot find those warnings

When I compile my document using tectonic, I see the message

warning: warnings were issued by the TeX engine; use --print and/or --keep-logs for details.

When I use the --keep-logs option and search for “warning”, “badness” etc. in the log, I cannot find anything. Furthermore, I tried this method for making warnings fatal and my document still seemed to compile fine.

I have two questions related to this situation:

  1. What is the warning in my case (link to log file)?
  2. In general, what method should I follow when tectonic tells me that warnings were issued by the TeX engine and I would like to identify what those warnings are?

PS: I am using bibtex.