Just saw this in the hotfix notes, Cascazia:
It was fixed when Blizzard said it was. I have managed to level up 2 already in BFA and brought in the 1 I leveled up in Legion due to the issue in to the BFA now.
Cool thanks, I already managed to get through all the quests now with a boosted character from my repurchase of shadowlands… However I’m stuck in Boralus now not getting the Mechagon Island intro quest. Can someone help me?
I know its a small gnome starting that whole questline to unlock the island but none of those npcs are here and I finished with Hall of Origination and Magni so it should come next right?
I only have A Tempered Blade left in Nazjatar that gives access to world quests there, I will try that, maybe its another tree of those quests before I get the NPC here…
by the way this is Cascazia from above, changed my character lol.
Update: The very last quest in Nazjatar I mentioned triggers the Gnome there and then you head back to Boralus, I made it! 
Hopefully this helps someone and that I get my race after the island again on this character since I already have the achievement.
So if I understand it correctly I need to hit the 50 lvl first, to obtain the heart? Seriously!!! This is ridiculous!!! What point to do the BfA quests without it? What did I even pay for???
I don’t know what you pay for, but I do know that you didn’t pay to be given everything straight away.
Yes, you not have to be level 50 to get the neck, but also the quests were made easier and the XP gains increased, to compensate.
You can now literally level from level 10 to level 50 in 16 hours.
Note that you also do not seem to get given Azerite pieces as a reward from doing quests if you are below level 50. IMO they really should have made this (neck & azerite) level 45 to emulate the BfA experience.
I don’t know if this helps, but I noticed on my lvl 40 chars that when i log in the first quest I am given is The Legion Return to start the legion content, if I get that quest then I can’t start the BFA content. If i dismiss the legion quest though, the Warchief’s Herald next to the Warchief’s Command Board in Orgrimmar has the quest to start the BFA content (talk to Silvanas, go to Stormwind etc etc).
I’ve been able to do the bfa content on all my alts starting al lvl 40 and then go back to Legion for the mounts no problem like this.
Hope it helps
I am level 53 and I am trying to unlock some Allied Races, but I am unable to since BFA appears to be totally bugged. I have gotten all 3 reps to friendly to unlock world quests, but I’m unable to unlock world quests. I cannot obtain the Heart of Azeroth, I cannot go to Mechagon or Nazjatar, and I’m getting replies from GMs that are copy pasted. Very disappointing customer service. If anyone can help or have had a similar experience, please let me know.
It appears to be due players being marked as “newcomer”, upon which you’ll need to play 30 hours /played time on that toon and then unlock all the things or level a new fresh toon from 1 to 50.
If you are still having any trouble after completing Battle for Lorderon via “I have already seen this…” - leave Stormwind and take a portal back. As soon as you are ported to Mage Tower in SW you should receive a quest to start the BFA questline.