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

[kfx]

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"