neutron-ovs-cleanup script cleans the ports from the integration bridge and then Nova (os-vif) will plug the relevant instances ports to it. This script doesn't run on ML2/OVN setups and it should for the same reasons it does on ML2/OVS.
afctully i dont think this is correct. even ml2/ovs should not have this run regually we dont expect this to be run upstream it ment as a tool to help fix leaked port when we hit bugs. this is racy to run on start up as it can interfee with nova/libvirt plugin the interface if the resume gusts on boot config option is enabled so i would suggest disabling this for both ml2/ovs and ml2/ovn by default.