IPv6 and uplink balancing

I have a question about ipv6 and uplink balancing.

I am using a HA UTM behind two CISCO routers, one for each Internet connection.

The routers are doing BGP with our own AS:

BGP seems to be working but after one link is down, the UTM tries to send IPv6 traffic still over the link which is down.

IPv4 is doing fine over the other connection but just IPv6 is not working.

UTM detects that one link is down of course.

Any ideas?