On old worlds, the exit portal was spawned under the dragon when it died. In most cases, this is somewhere else than 0,0. However, the respawning mechanism requires the portal to be at these coordinates.

Suggested fix: Allow respawning as long as there are 4 end crystals placed in the right pattern with at least one portal block in between them, then delete the old exit portal and place a new one at 0,0.