The Public IP is basically Dashboard's interpretation of how it communicates back to the device, and there are rare cases where it can be stale information. I just jumped into Postman and tried it on one of my own devices just to make sure it worked as expected and it did.
Assuming the MX is currently online in Dashboard (if you're getting a 200/OK response from the API call) what are you seeing for a public IP in the UI on the Appliance Summary page under the Uplink tab? Has the external/public IP recently changed from your ISP, or was this MX just brought up for the first time? It doesn't look like it now, but maybe before were you using (or changed) uplink preferences which could cause the Public IP to become stale?
If it doesn't clear in time, reboot the MX and check it again. If still not, I'd open a support ticket.