Currently doc_values are used in block loader only when FieldExtractPreference is DOC_VALUES https://github.com/elastic/elasticsearch/blob/main/server/src/main/java/org/elasticsearch/index/mapper/GeoPointFieldMapper.java#L536. With synthetic source, block loader from source will be used. However if doc_values are present, synthetic source will be built from those same doc_values. We should directly use doc_values in this case instead of the _source indirection.