I encountered a minor race (The Lantern) for the first time in my current game. I completed their quest (build them a drug factory). However I never got a culture point. Also after selecting my reward I pass the turn and find that when I talk to them again I'm still asked to select my reward.
Also not sure if this is related but in the same turn as completing the Lantern's quest I also completed the "upgrade missiles" mission and didn't receive any culture points for that either