ha backup router ipv6 accept_ra broken

When we restart the neutron-l3-agent we observe that backup routers start accepting router advertisements. This leads to routes inside the router namespace which expire.
e.g.:
$ ip netns exec qrouter-a5f7fb32-3e30-4e15-89f9-4ae888c2cac6 ip -6 r
x:x:1002:1::/64 dev qr-72f85121-ce proto kernel metric 256 expires 86355sec pref medium
x:x:1002:1::/64 dev qr-4e84792f-aa proto kernel metric 256 expires 86355sec pref medium
fe80::/64 dev ha-9d085c9d-15 proto kernel metric 256 pref medium
default via fe80::f816:3eff:fed3:3fa6 dev qr-4e84792f-aa proto ra metric 1024 expires 255sec hoplimit 64 pref medium
default via fe80::f816:3eff:fed3:3fa6 dev qr-72f85121-ce proto ra metric 1024 expires 255sec hoplimit 64 pref medium

When we now failover to such a backup router, the kernel does not create the necessary directly attached routes because they already exist. The problem is that those routes expire and because we are now a master router the routes do not refresh from the router advertisement anymore and expire after 24h which breaks ipv6 for those routers.

After we dug a bit deeper into this issue we found that the function [1] that disables the accept_ra on the backup routers always returns false. So backup routers never get their router advertisement disabled.

master router:
$ ip netns exec qrouter-92ed5c1f-c705-4ab9-a0e1-56e905d43abd sysctl net.ipv6.conf.qr-c7eb60ab-f1.accept_ra
net.ipv6.conf.qr-c7eb60ab-f1.accept_ra = 1

backup router:
$ ip netns exec qrouter-92ed5c1f-c705-4ab9-a0e1-56e905d43abd sysctl net.ipv6.conf.qr-c7eb60ab-f1.accept_ra
net.ipv6.conf.qr-c7eb60ab-f1.accept_ra = 1

[1] https://github.com/openstack/neutron/blob/stable/train/neutron/agent/l3/ha_router.py#L318