Picked up the Twilight Highlands quest from the call board in my garrison and then used my hearthstone to get back to Stormwind. Upon returning to Stormwind Keep, I was placed into the Shadowlands keep instance where there is no Anduin NPC to turn the quest into.
I have not tried to take the portal from Stormshield yet. My Garrison hearthstone is still on cooldown.
Edit: Took the portal. Still impossible.
I am also having this problem. I gave up on continuing in Vashj’ir after I couldn’t complete the Battlemaiden quest line, tried to do Twilight Highlands instead, and now I can’t continue because Anduin is missing and Turalyon is on the throne. 
EDIT: I abandoned Hero’s Call: Twilight Highlands! and went to speak to Chromie again to return to the present timeline. Then I went back to Stormwind Keep and was able to speak with Admiral Jes’terith and pick up Twilight Shores. I was then able to speak with Fargo Flintlocke to make my way to Twilight Highlands from there.
Thank you! Returning to the present timeline worked!!!
Thanks so much this worked !
thank you for this tip. im finally able to progress
My housemate reports that she had to do this too. patch 9.1.0 with 9.1.5 due to drop any hour now, seriously Blizzard PLEASE fix this.
I mean it’s bad enough that general randomness on the Hero Board is not random at all and you can’t sift through calls to get things from Zones you want to go to unless you have gone through so many that there’s less than 25 of them left.
But to have a Call in your log not result in phasing-in of the turn-in NPC is just insane.
Thank you! Returning to the present timeline worked!!!
We found that it didn’t work unless you BOTH ditched the Hero Call AND you went to Jes-Tereth instead. Ergo the Hero’s Call quest shouldn’t be put on the board, it’s a defunct quest; or, it should be fixed in phasing to be a working quest.