Did they nerf dungeon XP recently?

[Jeeqbit]

I'd get around a quarter of my bar worth.

On average, you should get about a third of your experience bar, in my "experience".

That is providing you are doing the leveling dungeon closest to your level. A leveling dungeon is any dungeon that is 1-49, 51-59, 61-69, 71-79 or 81-89 and excludes 50, 60, 70 and 80 dungeons.

I just ran Castrum Meridianum and got roughly five percent of my bar?

It's because it's a max level dungeon. These have never given much exp. If they did then it was probably a temporary bug.