Checked with version "4.6.0-0.nightly-2021-02-13-034601" setting "additionalNetworkIDs" parameter for master and worker nodes, installation went successful and bootstrap uses the same network context with master nodes as expected:

~~~
02-17 10:18:28  level=debug msg="OpenShift Installer 4.6.0-0.nightly-2021-02-13-034601"
02-17 10:18:28  level=debug msg="Built from commit 74b1d08f7f44c9c0d3f999c669a16fb2395e9551"
...
02-17 10:13:43  level=info msg="Obtaining RHCOS image file from 'https://releases-art-rhcos.svc.ci.openshift.org/art/storage/releases/rhcos-4.6/46.82.202011260640-0/x86_64/rhcos-46.82.202011260640-0-openstack.x86_64.qcow2.gz?sha256=2bd648e09f086973accd8ac1e355ce0fcd7dfcc16bc9708c938801fcf10e219e'"
...
02-17 10:14:16  level=debug msg="Image pamoedom-f7g25-rhcos was created."
~~~

~~~
$ openstack --os-cloud openstack server list --name pamoedom-f7g25
+--------------------------------------+-------------------------------+--------+-----------------------------------------------------------------------------------------+----------------------+-----------+
| ID                                   | Name                          | Status | Networks                                                                                | Image                | Flavor    |
+--------------------------------------+-------------------------------+--------+-----------------------------------------------------------------------------------------+----------------------+-----------+
| a80d5015-01d3-4d26-babd-09a41547c20d | pamoedom-f7g25-worker-0-fqkjl | ACTIVE | pamoedom-f7g25-openshift=192.168.2.236; pamoedom-network-test=172.16.1.144              | pamoedom-f7g25-rhcos | m1.large  |
| 1f36fae4-0a6b-42fa-a7d7-7d6290ddf3ac | pamoedom-f7g25-worker-0-zlxzf | ACTIVE | pamoedom-f7g25-openshift=192.168.3.178; pamoedom-network-test=172.16.1.165              | pamoedom-f7g25-rhcos | m1.large  |
| b75f8c8c-f874-4642-b294-876805f64061 | pamoedom-f7g25-bootstrap      | ACTIVE | pamoedom-f7g25-openshift=192.168.2.123, 10.0.102.89; pamoedom-network-test=172.16.1.188 | pamoedom-f7g25-rhcos | m1.xlarge |
| 82e525e3-38e7-4feb-9b81-903a21648324 | pamoedom-f7g25-master-1       | ACTIVE | pamoedom-f7g25-openshift=192.168.0.49; pamoedom-network-test=172.16.1.194               | pamoedom-f7g25-rhcos | m1.xlarge |
| 9863d775-7915-48be-a448-dc63ad1de17b | pamoedom-f7g25-master-2       | ACTIVE | pamoedom-f7g25-openshift=192.168.3.231; pamoedom-network-test=172.16.1.187              | pamoedom-f7g25-rhcos | m1.xlarge |
| 13fa8fa6-60e9-4f38-b622-de461d9c17a8 | pamoedom-f7g25-master-0       | ACTIVE | pamoedom-f7g25-openshift=192.168.1.209; pamoedom-network-test=172.16.1.167              | pamoedom-f7g25-rhcos | m1.xlarge |
+--------------------------------------+-------------------------------+--------+-----------------------------------------------------------------------------------------+----------------------+-----------+
~~~

Best Regards.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (OpenShift Container Platform 4.6.18 bug fix update), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2021:0510