Bug: Dovecot index loosing sync with FTS despite "fts_autoindex = yes"

dovecot 2.2.34 solr 7.2

I only see new messages after typing on the server "doveadm fts rescan -u username" though I've followed the wiki and added "fts_autoindex = yes" in 90-plugin.conf . Subsequent search for the same pattern always gives the same result, ignoring new emails with that particular pattern. Solr is working correctly and the correct search results comes immediately (whatever the searched word) after a "doveadm fts rescan"

# doveconf | grep fts mail_plugins = " quota stats fts fts_solr" fts = solr fts_autoindex = yes fts_solr = url=http://localhost:8983/solr/dovecot/ [POP]  mail_plugins = " quota stats fts fts_solr sieve virtual" [IMAP]  mail_plugins = " quota stats fts fts_solr imap_quota virtual imap_stats"