How to hide MAC during IPv6 forwarding?

Question

linux

Good afternoon.
I have Proxmox on Debian 9. With a typical network setup:

spoiler

auto lo
iface lo inet loopback
iface lo inet6 loopback
auto enp0s31f6
iface enp0s31f6 inet manual
auto vmbr0
iface vmbr0 inet static
address 99.999.99.118
netmask 255.255.255.192
gateway 99.999.99.129
bridge_ports enp0s31f6
bridge_stp off
bridge_fd 0
up route add -net 99.999.99.0 netmask 255.255.255.192 gw 99.999.99.129 dev vmbr0
iface vmbr0 inet6 static
address 2a01:aaaa:bbb:ccc::2
netmask 64
gateway fe80::1
auto vmbr1
iface vmbr1 inet static
address 10.10.0.1
netmask 255.255.255.0
bridge_ports none
bridge_stp off
bridge_fd 0
post-up echo 1 > /proc/sys/net/ipv4/ip_forward
post-up echo 1 > /proc/sys/net/ipv6/conf/all/forwarding
post-up iptables -t nat -A POSTROUTING -o vmbr0 -j MASQUERADE
post-up iptables -t nat -A POSTROUTING -s '10.10.0.0/24' -o vmbr0 -j MASQUERADE
post-down iptables -t nat -D POSTROUTING -s '10.10.0.0/24' -o vmbr0 -j MASQUERADE
iface vmbr1 inet6 static
address 2a01:aaa:bbb:ccc::5
netmask 64
gateway 2a01:aaa:bbb:ccc::2

Today, a letter of happiness arrived that I could candle the MAC addresses of virtual machines with IPv6 to the network and I should hide them.
Virtual machines are connected via vmbr0 bridge:

spoiler

iface eth0 inet6 static
address 2a01:aaa:bb:ccc:1:1:1:1
netmask 64
gateway 2a01:aaa:bb:ccc::2

What am I doing wrong?
tcpdump vmbr0 shows these MAC
On enp0s31f6 eh not visible