Comment 8 for bug 1793094

I think it may be an issue in horizon, because I don't see a way to reproduce that with CLI.

scenario #A:

Project -> Network -> Routers: Create router
 set name to 'test', select 'External network'

You have created a new router with a port in external network with IP from allocation pool.

scenario #B (this issue)
Project -> Network -> Routers: Create router
 set name to 'test', leave 'External network' empty.

Go to your new router:

Project -> Network -> Routers -> test:
 - switch to "Interfaces" tab
 - click "Add Interface"
Select Subnet (choose your external one), leave IP Address empty.
Submit and check what IP you got.

It should be from the allocation pool, but still it's trying to set to the gateway IP.