When searching on ISSN in Primo, not only journals are found, but also articles because articles in CDI have a data field ISSN.
This is not the case for locally catalogued articles because following the MARC standard, the ISSN of the journal is catalogued in tag 773$x.
Not only are locally catalogued articles not found when searching on ISSN, but also the facet on Journal title doesn't work for these local articles.
Therefor, our question is to add tag 773$x to the index on ISSN to allow for a consistent search experience for local articles and CDI articles.