Solr can't indexing relation list attribute when there is more than one object on every class, i got this error :

GRAVE: org.apache.solr.common.SolrException: ERROR: [doc=0198342842b84e01dc4bff468f2075f5] multiple values encountered for non multiValued field submeta_artists___always_available_b: [true, true]

Steps to reproduce

update solr index :
php extension/ezfind/bin/php/updatesearchindexsolr.php -s <admin
siteaccess> --php-exec=php --conc=2