I know, but I've learned to keep my expectations very low when it comes to updates on The West. And, to be honest, percentage xp wouldn't be good either, low level players would never catch the high level ones.
One could think of a reward based on a trivial BM25 function with a penalty based on level.
y = ((k+1)x / (x+k)) + x^c + p
where x = player_level, k and c are the main hyperparameters to adjust the penality and p = fixed XP reward.
Example:
set k to 1000, c to 1.6 and p to 500
lvl 10: 550 xp
lvl 100: 2150 xp
lvl 150: 4000 xp
Its a quick sketch of a possible solution. If they came up with the level up xp formula they are able to design such simple formula for the quests reward.
c around 1.8/1.9 is likely to match the level up xp curves but yeah, its just an idea