Just want to make it clear that I'm in no way attempting to down-play the severity of the XP-bug in my below post. I, just like others, am in the midst of my only two weeks vacation this year and am absolutely gutted over the problems the game has had since 1.04. That being said, I know some of you are still pussy-footing around the Northern realms afeared your levelling will hit its climax and you'll be forced to start from scratch. I sympathize with you, and that's the only reason I make this post..
I realize it's been reported as being the greyed out quests that specifically cause the XP bug to affect green, red, whatever future quests you run into. But I just did quick testing of my own, loading up an old save on PS4 (still patch 1.04) and ran Triss' secondary quest; A matter of Life and Death (suggested level 12), which was greyed out for me as Geralt's level 19 on that save. So after I gained some carnal knowledge from Triss and expectedly received no XP, I decided to do Dijkstra's secondary quest; Deadly Plot (suggested level 14), which was green in my journal, and surprisingly received 25XP. Although not much, I think it's safe for me to continue casually playing the game. Especially considering the fact that after running the first four main quests on Skellige (from a more recent, but still bugged save) and having not received any XP for them, I subsequently completed the fifth main quest and received all previously missed experience.. For which I have a working theory:
In short, I think the main quest Nameless may possibly prompt an XP gain for previous tasks, because completing it together with Family Matters, Ladies of The Wood, and The Play's The Thing, combine to trigger the main quest; Ugly Baby, as well as initiate the Second Act; The Great Battle. With that said, I think that in some way the 'sistering-quests', story-altering moments, and/or significant decisions made throughout may actually cause the game to generate the experience bugged in previous quests. And although theres no real way to confirm it without word from CD, or a thorough experience chart outlining each quest - I mean I could still be missing hundreds of XP points from past quests without knowing, I still do feel I'm in someway able to progress without being forced to restart..
So, as disappointing as some aspects of the game are right now, and as desperately as I want this fixed - its still playable. At least for me on the PS4.
Hope someone finds this helpful.