Thanks. I have been able to replicate your test on my own test account and it did indeed work in that instance. Sadly, however, not on the two real-life client lists where I needed it, which is odd and which I will have to investigate. I appreciate your effort.
I have opened a ticket with a screencast of my list / send group / re-broadcast findings. Maybe that gives a clue of where I went wrong.
As an idea:
You have this rather handy option to send a campaign to records that have opened / not opened a particular campaign right from within the campaign setup. Why can there not also be an option to send a campaign to previously sent / not sent of a specific campaign? That would be a much simpler UX and very intuitive for the particular scenario I am dealing with. Not to replace the send group feature, which I'm sure is great, but as a simpler workflow for the relatively common task of sending an email campaign to people who have just been added to a list etc.