See how org.hibernate.search.integrationtest.backend.tck.search.aggregation.SingleFieldAggregationBaseIT#multiValued is disabled due to org.hibernate.search.integrationtest.backend.lucene.testsupport.util.LuceneTckBackendFeatures#aggregationsOnMultiValuedFields.
Before HSEARCH-3839, we couldn't even index multiple values for numeric fields in Lucene. After HSEARCH-3839, we can, but we pick a single value when aggregating, so aggregations are still incorrect.
Ideally, when counting documents per field value, multi-valued documents should be counted once per value that appears in the field. So if a single document has values 1 and 2 for a single field, it should increment the count for both 1 and 2. At least that's what happens on Elasticsearch.
How to test the behavior on Elasticsearch:
So document 2 was counted twice.