Did they nerf dungeon XP recently?

[Astrus]

Castrum Meridianum is level 50 duty. Expansion max level dungeons always rewarded next to no exp. If you're in that level range, use either Aurum Vale or Dusk Vigil

Basically this.
Outside of the daily roulette bonus it has always been better to run the highest leveling dungeon available to you, level cap dungeons (50/60/70/80/soon 90) always had bad XP returns because at the level you used to run them at you didn't need the XP. Pretty much the same thing you see happening with post-expansion MSQ quests where the XP takes a nosedive until you reach the next expansion's x.0 MSQ.
As you say you're 79 Mt Gulg should be where you want to be leveling, until you get to 81 and can do the Tower of Zot - skip the level 80 dungeons.