In the DVR routers, neutron-server needs to populate ARP entries also for IPs added to the ports as allowed address pairs. When e.g. new IP is added to the allowed address pairs of the port, it works fine and neutron server sends notification about such new arp entry to the all L3 agents where dvr router is placed. But in case when new vm plugged to the same router is spawned on completly new compute, or existing vm is migrated to the new compute where dvr router wasn't created before, arp entries for allowed address pairs aren't populated at all.
Since the problem described in this bug report should be resolved in a recent advisory, it has been closed with a resolution of ERRATA. For information on the advisory (Red Hat OpenStack Platform 16.1.7 (Train) bug fix and enhancement advisory), and where to find the updated files, follow the link below. If the solution does not work for you, open a new bug report. https://access.redhat.com/errata/RHBA-2021:3762