I created a world on MCPE (Samsug galaxy) in 1.19, and defeated the dragon and obtained the egg. I later switched to PC (Dell Inspirion) and re-summoned and killed the dragon, a second egg appeared in the normal place. It behaves identically to the original and stacks with it. Whilst attempting to recreate the glitch in a creative copy, I found it worked the first time I killed the dragon, but not again.