I discovered this topic »How do I get debug or info logs from unbound-dhcp-leases-bridge app« the other day.
It seems, that some script unbound-dhcp-leases-bridge manages this dhcp-leases.conf file, which is obviously invoked by the dhcp service,
i. e. it seems, that the dhcp-leases.conf file is removed at dhcp startup.
So I did now additionally add a line to fcrontab:
@reboot [ -f /etc/unbound/dhcp-leases.conf ] || echo -e "# dhcp-leases.conf\012" > /etc/unbound/dhcp-leases.conf
This happens after dhcp startup and the file remained.