I'm at Commander 3 and did one Delta Volanis - no dilitium awarded.
Now looking at my missions I do not see dilithium being listed as a reward for:
I see it for Afehirr and Khazen...
My guess is that when you move up in level out of the exploration range the dilithium reward goes away?? If that is the case this is a further kick to exploration. If it is not, it is just sloppy on Cryptic's part.