Were you using poison or a damage over time ability and die just before diablo died?

It can happen in that type of scenario.

Once you leave the death screen, and end up in base you are no longer in the quest range.

It occurs when you are to far away to complete quest. This can occur if you are to far away in CS as well. The max range i am not sure, it might be 4 screen lengths away.

If you died remain dead till the damage over time has finished. You wont get the exp, but you will get the quest.