When we set l3_ha=true and gateway is not in exteranl network subnet cidr:

root@devstack-controller:~# neutron subnet-list |grep pub
neutron CLI is deprecated and will be removed in the future. Use openstack CLI instead.
| 941fd5a3-897e-4c24-9f26-0c51e3ced2e6 | public-subnet                                     | ebe65b5c2dc848e58e55b9640acf1279 | 192.168.5.128/25    | {"start": "192.168.5.130", "end": "192.168.5.254"}                            |
| d9bddf94-c4a6-47d2-b1ab-c30f2f85b177 | ipv6-public-subnet                                | ebe65b5c2dc848e58e55b9640acf1279 | 2001:db8::/64       | {"start": "2001:db8::3", "end": "2001:db8::ffff:ffff:ffff:ffff"}              |
root@devstack-controller:~# neutron subnet-show 941fd5a3-897e-4c24-9f26-0c51e3ced2e6
neutron CLI is deprecated and will be removed in the future. Use openstack CLI instead.
+-------------------+----------------------------------------------------+
| Field             | Value                                              |
+-------------------+----------------------------------------------------+
| allocation_pools  | {"start": "192.168.5.130", "end": "192.168.5.254"} |
| cidr              | 192.168.5.128/25                                   |
| created_at        | 2019-05-24T10:08:29Z                               |
| description       |                                                    |
| dns_nameservers   |                                                    |
| enable_dhcp       | False                                              |
| gateway_ip        | 192.168.4.129                                      |
| host_routes       |                                                    |
| id                | 941fd5a3-897e-4c24-9f26-0c51e3ced2e6               |
| ip_version        | 4                                                  |
| ipv6_address_mode |                                                    |
| ipv6_ra_mode      |                                                    |
| name              | public-subnet                                      |
| network_id        | d08a0c6f-27a9-4921-912b-c6884833c5da               |
| project_id        | ebe65b5c2dc848e58e55b9640acf1279                   |
| revision_number   | 2                                                  |
| service_types     |                                                    |
| subnetpool_id     |                                                    |
| tags              |                                                    |
| tenant_id         | ebe65b5c2dc848e58e55b9640acf1279                   |
| updated_at        | 2020-02-05T04:58:34Z                               |
+-------------------+----------------------------------------------------+

create a router and see ns route rules:

root@devstack-controller:/etc/neutron# ip netns exec qrouter-0dccc9f9-cf9f-4fa3-860f-7790401ddf89 route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
169.254.0.0     0.0.0.0         255.255.255.0   U     0      0        0 ha-96299f41-44
169.254.192.0   0.0.0.0         255.255.192.0   U     0      0        0 ha-96299f41-44
192.168.5.128   0.0.0.0         255.255.255.128 U     0      0        0 qg-5461e5d9-ad


It should has default route like "0.0.0.0         192.168.4.129   0.0.0.0         UG    0      0        0 qg-78bcb3cf-49" here.

And when l3_ha=false, the route rules in router ns:

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.168.4.129   0.0.0.0         UG    0      0        0 qg-78bcb3cf-49
192.168.4.129   0.0.0.0         255.255.255.255 UH    0      0        0 qg-78bcb3cf-49
192.168.5.128   0.0.0.0         255.255.255.128 U     0      0        0 qg-78bcb3cf-49

I test this in stein version, but I think it also has problem in master branch.