When you kill an Ender Dragon for the first time, it spawns an Dragon Egg on the pedestal, which can be collected, since the dragon can be revived without the Dragon egg. This works as intended. The issue is... If the egg is not collected from the pedestal, and you try revive Ender Dragon with an Egg still on top of the pedestal, the Dragon Egg dissapears forever.

How to reproduce:

• Create New world
• Enter The End and Kill Ender Dragon
• Collect Dragon Egg
• Craft Ender Crystals and revive Ender dragon

Result: You still have an Egg when the Dragon is revived

And then:

• Kill ender dragon
• Place Dragon egg back at the pedestal
• Revive Ender Dragon

Result: You lose Dragon Egg forever