[root@gsm-r5s9-01 ~]# nmcli  connection add type ethernet ifname eth1 con-name eth1* autoconnect no ipv6.method manual ipv6.addresses 2000::2/126 ipv6.route-metric 256 ipv6.routes '2806:aabb:abba:abab:baba:bbaa:baab:bbbb/128 src=2000::2'
Connection 'eth1*' (20f64cf2-c5ab-4faf-a9b1-24318db2b773) successfully added.

[root@gsm-r5s9-01 ~]# nmcli  connection 
NAME       UUID                                  TYPE      DEVICE 
testeth0   0b8248b9-4ce8-4c70-be70-f8130b38bfe6  ethernet  eth0   
lo         7b8f6f3a-4adb-49d2-b3bd-81c443d749a9  loopback  lo     
eth1*      20f64cf2-c5ab-4faf-a9b1-24318db2b773  ethernet  --     
testeth1   3c39f515-7a2a-4106-a0aa-7d99c30908a5  ethernet  --     
testeth10  e2a0eabf-dc31-4f08-ad84-7745b4a8c17f  ethernet  --     
testeth2   698f08cb-2de6-405d-9f7b-d93c21b9b907  ethernet  --     
testeth3   e3e140a8-5f93-4465-abc6-c9912fba0755  ethernet  --     
testeth4   a1c75056-4487-4f19-998a-62be9d9063f9  ethernet  --     
testeth5   c796e2a1-3d96-46b2-a024-74468dbe3e2f  ethernet  --     
testeth6   38ef9ace-cf7d-4c91-9e3e-07ca4b7eca46  ethernet  --     
testeth7   1f005512-5f3e-45f0-8f2e-61411b9f14df  ethernet  --     
testeth8   7c1b6b2c-f942-4cd4-bab1-2ad951face41  ethernet  --     
testeth9   c7ba9894-b1f7-41b2-839e-ca7c269421e9  ethernet  --     

[root@gsm-r5s9-01 ~]# nmcli  connection up id eth1* 
Connection successfully activated (D-Bus active path: /org/freedesktop/NetworkManager/ActiveConnection/5)

[root@gsm-r5s9-01 ~]# ip r
default via 10.16.123.254 dev eth0 proto dhcp src 10.16.122.97 metric 99 
default via 192.168.100.1 dev eth1 proto dhcp src 192.168.100.119 metric 100 
10.16.122.0/23 dev eth0 proto kernel scope link src 10.16.122.97 metric 99 

[root@gsm-r5s9-01 ~]# ip -6 r
2000::/126 dev eth1 proto kernel metric 256 pref medium
2620:52:0:107a::/64 dev eth0 proto ra metric 99 pref medium
2806:aabb:abba:abab:baba:bbaa:b dev eth1 proto static src 2000::2 metric 256 pref medium
fe80::/64 dev eth0 proto kernel metric 1024 pref medium
fe80::/64 dev eth1 proto kernel metric 1024 pref medium
default via fe80::2a8a:1cff:fe09:a5c1 dev eth0 proto ra metric 99 pref medium



Reproducible: Always

Steps to Reproduce:
1. nmcli  connection add type ethernet ifname eth0 con-name eth0* autoconnect no ipv6.method manual ipv6.addresses 2000::2/126 ipv6.route-metric 256 ipv6.routes '2806:aabb:abba:abab:baba:bbaa:baab:bbbb/128 src=2000::2'
2.nmcli con up eth0*
3.ip -6 r
Actual Results:  
2806:aabb:abba:abab:baba:bbaa:b dev eth0 proto static src 2000::2 metric 256 pref medium

Expected Results:  
2806:aabb:abba:abab:baba:bbaa:baab:bbbb dev eth0 proto static src 2000::2 metric 256 in ip -6 r output