It is suprising for me too, as I don't know about this problem on 16.04 LTS and I could not reproduce it. It was probably introduced in 17.04 or around that.
I have done some experimenting now and I managed to find out that the problem is caused only by profile for /usr/lib/dovecot/anvil (not dovecot profile itself). Also, adding just "singal," to the profile didn't work.
It is suprising for me too, as I don't know about this problem on 16.04 LTS and I could not reproduce it. It was probably introduced in 17.04 or around that.
I have done some experimenting now and I managed to find out that the problem is caused only by profile for /usr/lib/ dovecot/ anvil (not dovecot profile itself). Also, adding just "singal," to the profile didn't work.