If the ender dragon is killed in at least update 1.6, possibly later, the ender dragon will not respawn when end crystals are placed around the exit portal. Placing the crystals will result in nothing happening. In survival, the crystals will be returned to your inventory if you go through the portal, even if you placed them. Sometimes just clicking on the slot where the crystals were in your inventory brings them back if you are in the End. Blowing up the crystals will cause the typical explosion, but they will still return to your inventory upon leaving the End.

This is a link to a video with what I mentioned:

https://www.reddit.com/r/Minecraft/comments/frippu/does_anyone_know_why_my_end_crystals_are_doing/