The DC export plugin puts the ISSN currently in the dc:source field, which is used also for the journal title and issue information.
Some harvesters have a problem to parse that correctly. E.g. BASE (https://www.base-search.net), which is one of the largest search engines for academic articles, just appends the ISSNs using semicolons without qualifying them (example: https://www.base-search.net/Record/35eb9e5d88121b2b51b62ca7b430e0230209f3d005780e296fbac7dd9f81f29d/ ). The harvester for Primo library catalogs can’t parse the ISSN correctly.
The ISSN should be treated as an identifier and the dc:identifier field should be used with the qualified identifier prefix urn:issn: prepended to the ISSN instead of the dc:source field. This is done so by publication repositories in OAI-PMH.
The same holds true for the DOI, where a info:doi/ prefix should be prepended to the DOI.
There is an old Github issue " Support qualifications for DC identifiers in OAI interface" Support qualifications for DC identifiers in OAI interface · Issue #1386 · pkp/pkp-lib · GitHub by @asmecher for OMP which had been opened, but was not followed up. That could be valid for OJS, too.
Application version: OJS 3.3.0-10 (but also valid for all 3.x versions)