The gateway IPv6 address you are using is wrong, it is a link-local IPv6 address and local to each device and you cannot connect to it. Try to find the IPv6 of your unifi router, mine is fd88::1 for Openwrt router.
I can confirm that this works, on my Openwrt I can see that lease is expired on shutdown (correct way) and kept the DUID the same.
Proxmox staff needs to look into this and remove the line that removes dhclient files.
I checked and it is not configured for my VMs and they all have native IPv6 connectivity.
This is what I have in the config
[OPTIONS]
enable: 1
radv: 1
macfilter: 1
policy_in: ACCEPT
[RULES]
|GROUP deny-lan-access
I also have firewall enabled on the datacenter level and IPv6 is working fine for both Proxmox and VMs.
Are you also allowing router advertisements?
accept_ra 2
Example
I had the same problem with my external USB HDD passed to my homeserver VM (SMB) home file sharing and the HDD just would not go to sleep.
What I did was I went to my VM > Hardware > Add > USB Device > Use USB Vendor/Device ID > Then choose the USB device from the list, now the HDD goes to sleep...
Just read that slaac does not work on bridges - https://www.reddit.com/r/Proxmox/comments/oq2rgi/how_to_use_ipv6_slaac_and_dhcpv6_for_the_proxmox/
I would recommend you to use IPv6 tokens to set a suffix this way the prefix can change any time.
I also use dhcpv6, follow the guide here and see if it works for you. https://saudiqbal.github.io/Proxmox/proxmox-IPv6-interface-setup-DHCPv6-or-static.html
This is what I found in my logs, I still dont know after so many years why Proxmox changes the value of default-duid in /var/lib/dhcp/dhclient6.vmbr0.leases on every boot.
Dec 28 02:32:38 s02 info[892]: executing ip -o addr show vmbr0
Dec 28 02:32:38 s02 info[892]: vmbr0: dhclient: new address...
I wish there was a one time donation option like $25 just to remove the subscription warning and show support for Proxmox and keep the No-Subscription Repository.