Hey! I had the same issue with this character until I tried some of the dialogue options with Chromie and was able to pick a timeline this way:
“I have a question about Timewalking Campaigns.” → “What are Timewalking Campaigns?” → “What timeline should I choose?”
It’ll give you a vague list to pick from and when I went with “I want to be the coolest Warlock on Azeroth!” it gave me the starting quest for Legion. After a quick check the mobs scale to 59 again for me, so I hope it’ll work for you as well!