This might be a bug.
The Heart of Azeroth was working fine for me when i tried to change its essences today.
But when i tried that in the Shadowlands, it said “link to azerite was severed”. The same happened when i returned to Azeroth and i can’t change it anymore.
It seems to be a bug. And unfortunately, i don’t think Blizzard will correct it very soon.
*Gladly, i already had my BiS essences equiped.
*Edit: After logging off and returning after a couple of hours, essences are again able to be changed in resting areas in Azeroth. For the record, i logged in outside Shadowlands zones, in Chamber of Heart.
That is DEFINITELY a bug.