Comment 2 for bug 1131082

I checked the more detail condition to reproduce this issue.

Could you login with 'demo' user rather than 'admin' user?
In my test, this issue only happens when I logged in with 'demo' user.

In a regular user, a router gateway port is not seen to the user. It may be the cause.
If the network topology is constructed from a port list, it is reasonable that a link between ext_net and a router is not displayed. We can know an external network of a router from router['external_gateway_info'].get('network_id').

I hope it help you.