The reproducer for this is simply to boot from a netinst.iso, go into the 'Installation Source' spoke, change nothing - leaving the drop-down menu at its default 'Closest mirror' setting - and return to the hub.

On Alpha TC3 - with anaconda 18.6 - this behaves as expected. I boot to the hub, wait while it sorts itself out, and it shows 'Closest mirror' as the Installation Source. If I then go into the Installation Source spoke and return to the hub, as described, it does just what you'd expect - nothing changes, 'Closest mirror' is shown as the source, no error triangle.

On the pre-TC4 build tflink made - http://imagebuilder.fedoraproject.org/testimage/18/pretc4/20120822_f18alpha_pretc4.x64.iso - which includes anaconda 18.6.1, this is broken. When you boot to the hub and just leave it sitting there, the 'Installation Source' is listed as 'Closest mirror', with no error triangle - but if you go into the Installation Source spoke and leave it again with the drop-down set to 'Closest mirror', then the hub changes so Installation Source is listed as 'Failed to set up install source' with an error triangle, and Software Selection shows 'Installation source not set up'.

I wonder if this is somehow related to the traceback tflink sees when trying to actually proceed with a default 'Closest mirror' install - https://bugzilla.redhat.com/show_bug.cgi?id=851274 .