/usr/sbin/kresd -c /etc/knot-resolver/kresd.conf -v -f 4 -k /etc/knot-resolver/root.key /var/knot-resolver

I don't immediately see what happens in your case, but so far kresd does not support running under systemd supervision with multiple forks.  It's actually supposed to print a descriptive error and exit if that configuration is detected.  See also https://gitlab.labs.nic.cz/knot/knot-resolver/issues/85#note_50539

--Vladimir