Playing on PS4, but there's a lot of misinformation going around about this
By design: You don't receive experience from quests that are gray in your quest log, ie. 6 or more levels below yours. Sometimes you receive small amounts of exp for completing quest steps or using Axii in conversations, and apparently the official strategy guide says you're supposed to receive less experience for completing gray quests, instead of nothing - but whatever.
The glitch or bug: Receiving no experience from quests that are green or red in your quest log, ie. within the permissible range for exp rewards. Exactly which kind of quest is affected seems random. For me, it's everything: Main quest, side quests, contracts. Some people get exp solely from main quests, while others say they get no exp from main quests but sometimes from secondary.
When does it happen: Nothing has been confirmed. Theories include: Completing certain quests trigger it, completing gray quests trigger it, the most recent patch (PS4 - 1.03, deployed wednesday may 27th) introduced it. I noticed it happening in my game on friday, after I had hit level 25 and completed first batch of Kaer Morhen quests. Started a new game to test last night, and quests in White Orchard awarded no XP (so nothing in particular "triggered" the bug). Currently I'm leaning towards it being introduced with the patch, but completing quests you accepted during the previous patch rewarded exp as intended, while newly accepted quests don't. Hence why reports of the bug only became widespread near the weekend.
A lot of people are frustrated about the design decision to not reward gray quests, but that's not the bug and conflating the two doesn't help.