This topic was already discussed around March 1st 2019, without having received a clear outcome.

Now, dealing with ECMP with OSPF in Bird 2.04 on Fedora30, I run in same problems:

-> multiple next hops to same destination (50.50.50.0/24) are in the route table of bird:

bird> sho route

Table master4:
10.10.10.0/24   unicast [OIPv46 13:39:16.979] I (150/10) [10.10.10.1]
dev enp0s3
192.168.122.0/24   unicast [OIPv46 13:46:18.979] E2 (150/10/10000) [10.10.10.3]
via 10.10.10.3 on enp0s3
50.50.50.0/24   unicast [OIPv46 13:46:18.979] E2 (150/10/100) [10.10.10.3]
via 10.10.10.2 on enp0s3 weight 11
via 10.10.10.3 on enp0s3 weight 11
bird>
[root@localhost ~]#

but kernel route table looks unexpectedly different:
root@localhost ~]# netstat -rn
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
10.10.10.0 0.0.0.0 255.255.255.0 U 0 0 0 enp0s3
50.50.50.0 10.10.10.2 255.255.255.0 UG 0 0 0 enp0s3
192.168.122.0 10.10.10.3 255.255.255.0 UG 0 0 0 enp0s3
[root@localhost ~]#

Kernel looks configured correctly (CONFIG_IP_ROUTE_MULTIPATH=y), etc.etc.

Bird config on ECMPed bird router: protocol ospf OIPv46 {
tick 2;
rfc1583compat yes;
ecmp yes limit 8;
merge external yes;
ipv4 {
import all;
export all;
};
area 0 {
interface "enp0*" {
# type broadcast; # Detected by default
cost 10; # Interface metric
ecmp weight 11;

What is missing either in the bird-config or the kernel part?

Thx