While the ender dragon remains alive, placing end crystals on the end portal does not initiate the respawn of the ender dragon upon its death. Re-establishing the end crystals requires breaking them and then placing them anew.

steps to reproduce

1. Enter the end
2. Respawn the ender dragon if it has been previously defeated.
3. Position the four end crystals atop the end portal while the ender dragon is still active
4. Defeat the ender dragon

Observed: Despite the presence of end crystals on the end portal, no ender dragon is respawned.
Expected: The ender dragon should be resurrected upon its demise, given the presence of end crystals on the end portal.