To add a bit more of detail:

The behavior is related to neutron's "enable_snat_by_default" setting: 

- if it's set to "true" (which is the default value), then a newly created router will have "enable_snat" set to true (expected) and you can change that setting via CLI (expected)

- if it's set to "false", then a newly created router will have "enable_snat" set to false (expected) and you cannot change that setting via CLI (which is unexpected)


Below terminal log show the following sequence of commands and results:

- enable_snat_by_default is set to true (and neutron restarted, to use that setting)
- the router settings show "enable_snat": true (which was created previously)
- "neutron router-gateway-set --disable-snat" and "--enable-snat" is invoked on that router, without returning errors, and the router settings are positively changed
- enable_snat_by_default is set to false (and neutron restarted, to use that setting)
- "neutron router-gateway-set --disable-snat" and "--enable-snat" is invoked on that router, without returning errors. "disable" actually disables SNAT, while "enable" does not change back to enabled state.

--- cut here ---
control1:~ # grep enable_snat_by_default /etc/neutron/neutron.conf
enable_snat_by_default = true
control1:~ # systemctl restart openstack-neutron.service
control1:~ # neutron router-list
+--------------------------------------+----------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------+-------+   
| id                                   | name           | external_gateway_info                                                                                                                                                                      | distributed | ha    |   
+--------------------------------------+----------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------+-------+   
| cbc39730-34cc-4d18-986a-5b6b9b1b4e96 | ebl-router01   | {"network_id": "18db85e5-36aa-4669-9004-9ec43baad3f2", "enable_snat": true, "external_fixed_ips": [{"subnet_id": "dafed1c4-da5b-4557-bf18-cc7f9266f82a", "ip_address": "192.168.164.40"}]} | False       | False |   
+--------------------------------------+----------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------+-------+   
control1:~ # neutron router-gateway-set --disable-snat cbc39730-34cc-4d18-986a-5b6b9b1b4e96 18db85e5-36aa-4669-9004-9ec43baad3f2
Set gateway for router cbc39730-34cc-4d18-986a-5b6b9b1b4e96
control1:~ # neutron router-list
+--------------------------------------+----------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------+-------+  
| id                                   | name           | external_gateway_info                                                                                                                                                                       | distributed | ha    |  
+--------------------------------------+----------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------+-------+  
| cbc39730-34cc-4d18-986a-5b6b9b1b4e96 | ebl-router01   | {"network_id": "18db85e5-36aa-4669-9004-9ec43baad3f2", "enable_snat": false, "external_fixed_ips": [{"subnet_id": "dafed1c4-da5b-4557-bf18-cc7f9266f82a", "ip_address": "192.168.164.40"}]} | False       | False |  
+--------------------------------------+----------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------+-------+  
control1:~ # neutron router-gateway-set --enable-snat cbc39730-34cc-4d18-986a-5b6b9b1b4e96 18db85e5-36aa-4669-9004-9ec43baad3f2
Set gateway for router cbc39730-34cc-4d18-986a-5b6b9b1b4e96
control1:~ # neutron router-list
+--------------------------------------+----------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------+-------+   
| id                                   | name           | external_gateway_info                                                                                                                                                                      | distributed | ha    |   
+--------------------------------------+----------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------+-------+   
| cbc39730-34cc-4d18-986a-5b6b9b1b4e96 | ebl-router01   | {"network_id": "18db85e5-36aa-4669-9004-9ec43baad3f2", "enable_snat": true, "external_fixed_ips": [{"subnet_id": "dafed1c4-da5b-4557-bf18-cc7f9266f82a", "ip_address": "192.168.164.40"}]} | False       | False |   
+--------------------------------------+----------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------+-------+
control1:~ # vi /etc/neutron/neutron.conf
control1:~ # grep enable_snat_by_default /etc/neutron/neutron.conf
enable_snat_by_default = false
control1:~ # systemctl restart openstack-neutron.service
control1:~ # neutron router-list
+--------------------------------------+----------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------+-------+
| id                                   | name           | external_gateway_info                                                                                                                                                                      | distributed | ha    |
+--------------------------------------+----------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------+-------+
| cbc39730-34cc-4d18-986a-5b6b9b1b4e96 | ebl-router01   | {"network_id": "18db85e5-36aa-4669-9004-9ec43baad3f2", "enable_snat": true, "external_fixed_ips": [{"subnet_id": "dafed1c4-da5b-4557-bf18-cc7f9266f82a", "ip_address": "192.168.164.40"}]} | False       | False |
+--------------------------------------+----------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------+-------+
control1:~ # neutron router-gateway-set --disable-snat cbc39730-34cc-4d18-986a-5b6b9b1b4e96 18db85e5-36aa-4669-9004-9ec43baad3f2
Set gateway for router cbc39730-34cc-4d18-986a-5b6b9b1b4e96
control1:~ # neutron router-list
+--------------------------------------+----------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------+-------+
| id                                   | name           | external_gateway_info                                                                                                                                                                       | distributed | ha    |
+--------------------------------------+----------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------+-------+
| cbc39730-34cc-4d18-986a-5b6b9b1b4e96 | ebl-router01   | {"network_id": "18db85e5-36aa-4669-9004-9ec43baad3f2", "enable_snat": false, "external_fixed_ips": [{"subnet_id": "dafed1c4-da5b-4557-bf18-cc7f9266f82a", "ip_address": "192.168.164.40"}]} | False       | False |
+--------------------------------------+----------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------+-------+
control1:~ # neutron router-gateway-set --enable-snat cbc39730-34cc-4d18-986a-5b6b9b1b4e96 18db85e5-36aa-4669-9004-9ec43baad3f2
Set gateway for router cbc39730-34cc-4d18-986a-5b6b9b1b4e96
control1:~ # neutron router-list
+--------------------------------------+----------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------+-------+
| id                                   | name           | external_gateway_info                                                                                                                                                                       | distributed | ha    |
+--------------------------------------+----------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------+-------+
| cbc39730-34cc-4d18-986a-5b6b9b1b4e96 | ebl-router01   | {"network_id": "18db85e5-36aa-4669-9004-9ec43baad3f2", "enable_snat": false, "external_fixed_ips": [{"subnet_id": "dafed1c4-da5b-4557-bf18-cc7f9266f82a", "ip_address": "192.168.164.40"}]} | False       | False |
+--------------------------------------+----------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------+-------+
control1:~ # 
--- cut here ---

This is 1005 reproducible.

Expected behavior: Even if "enable_snat_by_default" is set to "false", indivual routers' SNAT setting should be changeable.