After I followed the guide at https://docs.openstack.org/install-guide/launch-instance-networks-provider.html the interfaces started to show up in brctl show.
Originally I had created the networks through the Horizon Web UI and the networks I had configured in ml2_conf.ini and linuxbridge_agent.ini, were not assigned to the networks that were created through the webui.
I discovered this by running
openstack network list
openstack network show ID
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