There's already a call out to FOLIO to improve how multi-volume holdings appear in XML to make them visible via Z39.50, but it would probably help matters if Atlas Systems and FOLIO worked together to make this improvement for all libraries migrating to FOLIO. https://issues.folio.org/browse/ZF-84

Basically, if there are multiple volumes of a title, only summary holdings and inaccurate availability status are visible through Z39.50. Some libraries have implemented custom addons to get around this, but this is really a workaround to what should be a standard function.