Dovecot frequently full-text reindexes the whole maildir

[vitalif at yourcmc.ru]

Hi!

I'm using Dovecot 2.2.13 (Debian Jessie package 1:2.2.13-12~deb8u1) on 
my personal mail server (the address I'm writing from is on this 
server).

I use Maildirs, I have fts + fts_squat enabled, and I have a problem 
with it for a long time - dovecot seems to not update the index always 
"incrementally".

Yesterday I've finally made a test by telneting to imap port and issuing 
a search request. Dovecot started to reindex the mailbox. Full 
reindexing has taken around 15-20 minutes.

After reindexing the fulltext searches were fast, just as they should 
be. And they were fast yesterday even after some new emails arrived.

But I've tried to do a new search today and Dovecot started to reindex 
the whole maildir again!

Why is it reindexing the whole maildir again? Could it be related to my 
anti-spam scripts removing messages from SPAM imap folder? And if yes, 
how to remove them correctly?

-- 
With best regards,
   Vitaliy Filippov