Comment 1 for bug 2035230

IP allocation might be deferred in a routed network configuration where it is unknown where the port is going to "live" since we don't want to assign an IP that will not work on a particular compute node.

This state is normally shown in the POST response, but I believe you have to have the ip_allocation extension loaded.

This document has more information:

https://docs.openstack.org/neutron/latest/admin/config-routed-networks.html

That is the most likely reason fixed_ips is empty.

You can check in /etc/neutron/plugins/ml2/ml2_conf.ini to see what extension_drivers is set to, if you add this extension and restart neutron-server the API call should have that field (and value).

That's where I would start.