For starters,
I guess this refers to people who do all the side quests and the world investigation,thus should have found by now that even a slightest turn off the main story road yeild so much exp that you will outlevel main story quest req. very quickly.
So,this bug wont be a big deal for players.
Now,what i suggest as a temp. solution:
If you are do not get any exp at all for all the quests or feel cheated in someway for not getting what you have deserved,even if its just a side quest and do not want to stop playing this weekend you may try this:
1) If you already experienced this bug,evaluate its net loss. (This should have proably started may 25 for pc players).
2) Google Witcher 3 mods,find "Debug Console Enabler" (nexusmods should be good and virus free option).
3) Install it.
4) Open the console and type in "addexp (your evaluated number)".
5) This shall give you all the exp you lost (dont missuse it tho,we are for fair play here
) so you can continue with your playthrought this weekend.
6) If you encounter this bug again,u can simply google the quest you did,watch how much exp it yeild to other player and add this amount again throught the cosole.
This way you wont have to go back in the game and can continue your epic adventure this weekend,untill the fix is in place.
If any one here can provide a database with all the quests and thier exp rewards,that would save us a lot of time =)