Set a static address part with IPv6 and NetworkManager

In /etc/netplan/00-<interface>.yaml configure a ipv6-address-token like this:

sudo nmcli con mod 'Common net' ipv6.addr-gen-mode eui64 
sudo nmcli con mod 'Common net' ipv6.token ::252

Then restart the interface if needed:

sudo nmcli con down 'Common net'; sudo nmcli con up 'Common net'