The current way of retrieving titles from erm is not optimal, as the way its currently implemented in EusageReportsApi.java#L497 is not searching specifically for a resource with an ISSN of 1557-7341 but actually for a resource with an identifier of 1557-7341 where that resource has at least one identifier in the ISSN namespace.
That is - these two criteria would act separately then be combined with AND. This is a subtle difference of course and in most cases will work for ISSN because their format is so specific.
Instead the following contruct should be used:
/erm/titles works exactly the same as /erm/resource but will only search titles (where /erm/resource searches both titles and packages together). The request parameters and response format are exactly the same
The support for the new grouping syntax in search is only available with 6.0 upwards