The bibliography is generated (by core gramps) before the report code is executed. That biblio does not include the citation if there is nothing defined in the citation in the date or page/volume fields.
The report then uses the information in the biblio to generate the report. As example, look at the Person view Citation tab. If there is an event that has a citation without date or page/volume, the citation is not included in the Person view Citation tab (see the “<No Citation>” in the furthest back window).
So this cannot be fixed in the report (other than noting this requirement). It is in core gramps, which is beyond me.