Thanks for your post, @Borris1974. It was very helpful.
For future reference, for our Promox 7.4 cluster using dedicated servers and vSwitches on Hetzner, with multiple vmbrX bridges, we used the following /etc/sysctl.d/99-proxmox-arp.conf:
#...
Hardware and Software
Proxmox VE 9.1.6
Fujitsu T935
Intel Corporation Ethernet Connection (3) I218-LM (rev 03)
Kernel driver: e1000e
Interface name: nic0 (yours may differ -- typically eno1 or enp3s0)
Guest VM: Any (the fault is at the host NIC...