When you place the crystals while the ender dragon is dying then the dragon won't be resummoned, even after the current dragon's death animation has finished. It will only summon it again once a player places another crystal anywhere in the world. This makes no sense.

I would expect the game to resummon the ender dragon after the old one has died if there are ender crystals in the right spots.

Steps to reproduce:

1. kill the dragon
2. place crystals to resummon the dragon during the dragon's death animation
3. wait until the death animation finishes
4. the dragon won't be resummoned

Some people enjoy killing the dragon and want to do it as many times as possible.