Hi, I think I spotted the problem. It's difficult to be sure, but apparently, since version 4.31, latexmk automatically detects and run biber as a citation engine (see this post https://tex.stackexchange.com/a/231351/259874). In fact, in the latexmk man page, it is said to use the option -bibtex to use BibTeX.
Indeed, if I try to run "latexmk -pdf -bibtex -f" (the -f option is needed due to some errors given by whitespaces in the citations), it all works fine. From my point of view, the problem here is that the variable org-LaTeX-pdf-process doesn't track any of this! + If the variable cite_export is set to basic or to csl, the formatting is external to LaTeX, so no problem. + If it is set to biblatex, then biber is automatically called by latexmk + If it is set to natbib, LaTeX needs BibTeX to be called, but it is not! So I'd say this is a bug of the org-LaTeX-pdf-process variable. Should it be buffer-local, and set when exporting? This is not something I'm expert of, to be honest. As I was saying, using AuCTeX there's no problem, because the compilation command is an elisp function that checks which one of the two is needed. May that be helpful? Alessandro