Comment 5 for bug 1794478

Indeed, sorry about that, I overlooked that this was on a bond and tested with a bridge instead.

Here are the results with a bond:

$ cat /etc/netplan/50-cloud-init.yaml
network:
    version: 2
    ethernets:
        eth0:
            dhcp4: false
    bonds:
        bond0:
            interfaces: [eth0]
            dhcp4: yes
            addresses: ["fe81::32a5:bc5f:287f:8db8/64"]
$ ip addr show dev bond0
2: bond0: <BROADCAST,MULTICAST,MASTER,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether a2:83:94:5d:cc:de brd ff:ff:ff:ff:ff:ff
    inet 10.141.225.183/24 brd 10.141.225.255 scope global dynamic bond0
       valid_lft 3540sec preferred_lft 3540sec
    inet6 fe81::32a5:bc5f:287f:8db8/64 scope link
       valid_lft forever preferred_lft forever
    inet6 fe80::a083:94ff:fe5d:ccde/64 scope link
       valid_lft forever preferred_lft forever
$

Again, the dhcp4 address and the static ipv6 address are both present.