After killing Eikthyr we (four players) got the Dream. And today, several ingame days later, we killed the Elder, got the achievement, placed the trophy at spawn and got the text pop-up "You will dream tonight.", but the video didnt play, we theorized that it might not have played because we went to bed in the middle of the day.. So we waited untill nightfall the other day. But did not get it to play that time either.

So maybe a bug?

Hard Mode- no additional world settings, pure vanilla.