pfSense

On a system where it is not giving the "user unbound does not exist" message, the problem can be exhibited by:
unbound-control -c /var/unbound/unbound.conf reload
ok

Then unbound dies, system log has:
Mar 12 15:04:12 kernel: pid 33159 (unbound), uid 59: exited on signal 11

This is the fault symptom logged by that link above to the nlnetlabs bug report. It seems an easy way to exhibit the fault, and thus to test it is fixed on a new build with Unbound 1.5.3

that fixed unbound user not found for me

fixed for me also