This exact thing is happening to my fresh lvl 50 Zandalari.
I leveled only through Voldun in Zandalar while in Chromie time, the rest was different stuff in Chromie time and regular Cataclysm and WotLK zones before the prepatch.
Also, Sylvanas wanted to send me to free Tanalji after hitting 50 and getting the Heart of Azeroth. Remove the power form the Heart until you reach 50 or do what you have to do but getting asked to start the BfA campaign after leveling in BfA is weird… This might just be a bug as the Tirisfal issue probably is.
Anyway, BfA should start with the skippable Battle for Lordareon Scenario then freeing Talaji and getting the Heart right away - as is is a significant portion of the story campaign of BfA. It can be different for newbies from Exile’s Reach but not for Veterans… But this is a different issue.