So I executed hugo server to checkout an older project of mine and lo and behold:
ERROR 2019/06/04 22:18:09 Symbolic links for directories not supported, skipping "/assets/some-directory"
This used to work in previous versions of Hugo but not anymore. At some point something changed. Don’t know if it was for security reasons or something else.
In any case, I don’t quite mind this regression, since this is a project that needs to be rewritten from scratch.
However I am posting about this here for posterity, because others may encounter the same issue.