Daily Chaldea - Ch. 2665 - Beyond The Impossible

[MangaDex]

 

[Boring-Ash]

Proof it is a alternate universe

 

[LarousseBR]

Only Cinderella Elizabeth have permission to sing.

 

[Malachite_Dragon]

The actual reason is, when she's singing for herself/selfish reasons, she sounds god-awful. When she's singing for others, it sounds good. Yes, this is the actual reason and it was discovered in-game during a Halloween event.