I have the same problem (my logs look the same) and are very interested
in a solution.
I was wondering which version of OpenBSD are you using? and which version of
Dovecot?
Fully syspatched and 'pkg_add -u'ed
OpenBSD 6.5
OpenSMTPD 6.5.0
dovecot-2.3.9.2v0
I usually upgrade in a timely manner to the latest stable, but due to
circumstances I'm still on 6.5.
Have you found a solution in the mean time?
Haven't found time to investigate further. I hope to find time in a view
weeks to upgrade and look again into this topic. Will post an update if
I find a solution.