I realize that doing a quest that is more than five levels lower that your current level results in only a couple of XP points gained. However, I have just completed two side quests that are four levels below mine, namely "A portrait of the Witcher as an old man" and "Of sheers and a Witcher I sing" and I received no points on completing either of them.

This is how the game was build, if you are too level up vs the quest recommended, you will get max 4-5xp, I did a level 11 quest with my character level 14, received 3xp...

There are a certain number of (side-)quests in TW3 that don't reward Geralt with experience points upon completing them - they often give XP for finishing certain quest stages, though.

The one about gathering a black pearl for Nidas is one of these (quest "Black Pearl").