In DSpace 6.x we migrated from integer-based IDs to UUIDs. Unfortunately, we neglected to update the Solr Statistics Schema based on this change (see also
While Solr v4 (used by DSpace 6.x) doesn't seem to validate all queries against the schema.xml, later versions of Solr have strict validation (so we cannot easily upgrade Solr, as required in DS-3695).
Additionally, as noted in the comments of
DS-3602, this incorrect schema may cause odd sorting behaviors in DSpace 6.x (as it attempts to treat UUIDs as integers).
The schema should be updated and a migration script (see https://github.com/DSpace/DSpace/pull/1810) created.