According to the ticket https://jira.duraspace.org/browse/DS-1483
The "citation_pdf_url" is now provided as follows:
--if the item has only one bitstream in the ORIGINAL bundle, link to this one
--if the item has a primary bitstream in the ORIGINAL bundle, link to this one (regardless of how many bitstreams there are in total in the ORIGINAL bundle).
--Bitstream must be publicly visible to be linked to in the "citation_pdf_url". The MIME Type of the bitstream is no longer checked, as Google Scholar has reported this is no longer important (see description of this ticket).
But when an item has more than one bitstream but no primary and all of them are public the "citation_pdf_url" will be null