My league has the same problem. <50 still have old quests. This is on both Dark Plains and Untamed Lands.
If gaining level 50 and higher does not grant the new 'easier' quests, please do NOT fix this bug...except on an individual case by case basis. At least then, it will be a choice, rather than a mandate.
Sure, the quests will be more difficult than the new ones, but at least those players can still get 160 free sketches per month once they reach level 80, vs the 80 free sketches per month the rest of us >=80 players are now stuck with after the nerf.
They will also be able to gain 5k experience per MQ experience quest (>=80), as well as have the opportunity to gain an extra 200 sapphires per month by finishing in the Top 10 of a league quest each week.