Reproduced, and yeah well that explains why I missed it .
I would have been re-configuring the same one over again I expect.
Workaround till we figure out what's going on:
1. Create the multi branch project
2. Add the SCM configuration
4. Click configure
5. You can now add behaviours
Ok, I found what is likely the cause, there's 3 table elements in the page when you add the github SCM
1. Discover branches behaviour
2. Discover pull requests from origin
3. Discover pull requests from forks
and I haven't been able to find where those are coming from =/
and after saving the table elements are gone and they are in their proper elements, weird...
From what I can tell it's not the github branch source plugin causing it, each plugin installed needs to be checked, (I don't have time right now to look at it any more)