I have received following suggestion from developers:
Please try next steps:
1. Exclude postfix-queue proxy from /etc/postfix/master.cf:
find string "smtp inet n - - - - smtpd -o smtpd_proxy_filter=127.0.0.1:10025"
and change a port 10025 -> 10026
2. monitor a log file /usr/local/psa/var/log/maillog and grep on strings that contains smtp status like: "lost connection after DATA". In most cases such strings must to be exists. This strings are means what connection was closed by native smtpd server and before-queue processor was closed by smtpd. It's normal. And may to be caused by wrong smtp clients. (e.g. spambots).
3. Let me know if logfile is clean from similair records. Possible it is bug in before-queue.