Fourth person with the same issues others are experiencing. No way to rejoin a previous covenant.
The two other threads with this issue is :
https://us.forums.blizzard.com/en/wow/t/i-cannot-change-my-covenant-to-nightfae/753293/
https://us.forums.blizzard.com/en/wow/t/game-breaking-cannot-swap-back-to-old-covenants/751058/