By 
CD Projekt Red say they’re working on the experience point glitch players have uncovered in The Witcher 3, and will be releasing a hotfix for the PC version on Monday.
I’m not sure I’ve run into this bug personally, but that’s sort of the point because it seems to strike at random.
As far as I can ascertain from reading other people’s reports, the glitch is different from the situation where you get no experience points for completing quests that are rated six levels (or more) below your own. That’s apparently intended game behaviour. The bug/glitch is when you should be getting experience points from a quest but it’s just not given to you. It’s not entirely clear whether this is just main quests, or side stuff like contracts as well.
In the post confirming the hotfix, CD Projekt say this bug “doesn’t impact the game balance in any significant way” (which is kind of weird, if it is indeed affecting a bunch of main quests.) If you’re concerned about whether this glitch is affecting you, there’s not much to be done except wait until Monday’s patch.
“Rest assured, we won’t stop working on improving [The Witcher 3] and there are more patches coming in the near future,” CD Projekt conclude.
AMD users who missed the news yesterday might also like to know that the 15.5 Catalyst beta drivers (with Witcher 3 performance boosts) are now available. Nvidia players with Kepler-based cards are still waiting on a fix from the company.