The Reaper rule sheet says:

"When playing with the Warlock Quest Cards, when a character lands on the Warlock’s Cave space, that player chooses freely one of the available Warlock Quest Cards from among those available in the Warlock Quest Card deck, instead of rolling a die to determine which quest he is assigned."

The Frostmarch expansion rule sheet says:

" In the base game, characters who travel to the Warlock’s Cave may roll a die to receive one of six quests. When playing with the Warlock Quest Cards, players must draw the top card from the deck of available Warlock Quests instead of rolling a dieto determine which quest he is assigned" 

I can´t find any clarification in the faq:s about this. Thanks!

After adding the Frostmarch expansion Quests to the game, the Quests selection becomes random.

Frostmarch is a later expansion. That's why the rules changed. Though we've always drawn Quests randomly, I believe the rules officially changed with Frostmarch, because this expansion adds easier Quests to the mix, such as Gain 1 Craft, or Gain 1 Strength. Thus the choice of Quests  was taken away from the player.

And of course, more cards were added to the Quests deck(now you have 36). So shuffling through all of them using the Reaper rule can get tiresome. Hence, selection was  randomized.

Before the expansion, you had to roll on the chart at the warlocks cave, and you could use a fate to re roll if you got a quest you didn't want. Does anybody play a style where you can use fate to draw a second card that you must take?