In DSpace 5, some fields in the solr-based usage statistics were switched over to have docValues="true" (https://github.com/DSpace/DSpace/commit/8e2f87e75548b48ad44c6257b47bf45af3e5b4ef). DocValues, added in Lucene 4, promise performance improvements for faceting data, an operation that is used very often in the usage statistics, eg to create downloads by country / downloads by city information.
As the official DocValues documentation (https://cwiki.apache.org/confluence/display/solr/DocValues) states,
"If you have already indexed data into your Solr index, you will need to completely re-index your content after changing your field definitions in schema.xml in order to successfully use docValues."
If existing data is not re-indexed, only those solr documents will be included in the faceting that were added after the schema change. For DSpace, this means that usage stats sections such as "downloads by city" will count only those downloads that happened after the upgrade. Older downloads are still stored in the solr statistics core but cannot be retrieved by faceting.