We’re experiencing the same issues. The certificate modified times correspond with the dovecot crashes to the second.
The way I determined this is by doing the following:
$ find /home -maxdepth 2 -type f -name "ssl.cert" -printf "%T@ %Tc %p\n" | sort -n
$ grep "master: Warning: Killed with signal 15" /var/log/syslog
I’ll be watching this topic for updates.