Can't breed nor spawn tamed trader llamas

When trader lamas are released from the wandering trader and tamed, we can give hay bale to two of them :

• They will go in love mode
• Seek for each other
• Get near and create a baby trader lama which instantly despawn.

==> You get the advancement "The Parrots and the Bats" but no counter is added on the "Two by two" one.

+ In Creative mode, the trader lama egg can be selected but the lama spawned will instantly disappear.

 

Is it a game feature that they can't be spawned without a wandering trader ?
-> If yes then why can we breed them and have access to their spawn egg ?