I'm not able to reproduce this using the resource_mapping policy driver. See below. I'm changing the status to incomplete.

[rkukura@localhost devstack]$ neutron router-create r1
Created a new router:
+-----------------------+--------------------------------------+
| Field                 | Value                                |
+-----------------------+--------------------------------------+
| admin_state_up        | True                                 |
| external_gateway_info |                                      |
| id                    | cf3e2943-89f6-4d0f-829d-3b76c1903886 |
| name                  | r1                                   |
| routes                |                                      |
| status                | ACTIVE                               |
| tenant_id             | 06bc0332286e4287914202f1521fb271     |
+-----------------------+--------------------------------------+
[rkukura@localhost devstack]$ gbp l3p-create --routers cf3e2943-89f6-4d0f-829d-3b76c1903886 l3p2
Created a new l3_policy:
+----------------------------+--------------------------------------+
| Field                      | Value                                |
+----------------------------+--------------------------------------+
| description                |                                      |
| external_segments          | {}                                   |
| id                         | 2f4c3579-6e30-4160-89ad-a338ec14bbbb |
| ip_pool                    | 10.0.0.0/8                           |
| ip_version                 | 4                                    |
| l2_policies                |                                      |
| name                       | l3p2                                 |
| proxy_ip_pool              | 192.168.0.0/16                       |
| proxy_subnet_prefix_length | 29                                   |
| routers                    | cf3e2943-89f6-4d0f-829d-3b76c1903886 |
| shared                     | False                                |
| subnet_prefix_length       | 24                                   |
| tenant_id                  | 06bc0332286e4287914202f1521fb271     |
+----------------------------+--------------------------------------+
[rkukura@localhost devstack]$ gbp l2p-create --l3-policy l3p2 l2p1
Created a new l2_policy:
+----------------------+--------------------------------------+
| Field                | Value                                |
+----------------------+--------------------------------------+
| description          |                                      |
| id                   | f32b98c5-352a-4bbe-bc31-10fcca3f2941 |
| l3_policy_id         | 2f4c3579-6e30-4160-89ad-a338ec14bbbb |
| name                 | l2p1                                 |
| network_id           | a47cb05e-8bf4-40a9-a6e3-e3ad8edbc920 |
| policy_target_groups |                                      |
| shared               | False                                |
| tenant_id            | 06bc0332286e4287914202f1521fb271     |
+----------------------+--------------------------------------+
[rkukura@localhost devstack]$ gbp ptg-create --l2-policy l2p1 ptg1
Created a new policy_target_group:
+---------------------------+--------------------------------------+
| Field                     | Value                                |
+---------------------------+--------------------------------------+
| consumed_policy_rule_sets |                                      |
| description               |                                      |
| id                        | a4a38d5e-dcb2-4573-bbaa-4d6ca0dc79f9 |
| l2_policy_id              | f32b98c5-352a-4bbe-bc31-10fcca3f2941 |
| name                      | ptg1                                 |
| network_service_policy_id |                                      |
| policy_targets            |                                      |
| provided_policy_rule_sets |                                      |
| proxied_group_id          |                                      |
| proxy_group_id            |                                      |
| proxy_type                |                                      |
| service_management        | False                                |
| shared                    | False                                |
| subnets                   | 666117d8-27e6-46a6-9fc1-e624781cec6b |
| tenant_id                 | 06bc0332286e4287914202f1521fb271     |
+---------------------------+--------------------------------------+
[rkukura@localhost devstack]$ neutron router-list
+--------------------------------------+------+-----------------------+
| id                                   | name | external_gateway_info |
+--------------------------------------+------+-----------------------+
| cf3e2943-89f6-4d0f-829d-3b76c1903886 | r1   | null                  |
+--------------------------------------+------+-----------------------+
[rkukura@localhost devstack]$ neutron router-port-list r1
+--------------------------------------+------+-------------------+---------------------------------------------------------------------------------+
| id                                   | name | mac_address       | fixed_ips                                                                       |
+--------------------------------------+------+-------------------+---------------------------------------------------------------------------------+
| 3e9c8fe4-fc03-45b3-b26c-5ea14eff4c2b |      | fa:16:3e:0a:19:b0 | {"subnet_id": "666117d8-27e6-46a6-9fc1-e624781cec6b", "ip_address": "10.0.0.1"} |
+--------------------------------------+------+-------------------+---------------------------------------------------------------------------------+
[rkukura@localhost devstack]$