As the Romulans asked it, I took the opportunity to declare war onto the Cardassians. For the most part to get a hold of the Bandi, which were right at my borders. As it's the nature of the Cardassians, they had taken the Bandi by force, so I needed to liberate the system.
So far so good, the Bandi thanked me for taking steps which fit into their future plans...but they are not available anymore as a Minor Race in the diplomatic screen. Their system shows up as not occupied by anyone, they're still there but are not available for diplomatic interactions.
I can remember from a long time ago, that I ran into this problem...do I have to take them by force or did I miss something?