If your spawnpoint is outside of a custom /worldborder barrier, then you cannot respawn after death. If you go to Title Screen and reopen the world, you will be where you last were the last time you saved. (before you died)

(Respawn works fine if your spawnpoint IS within the worldborder

Reproduce:
1. /spawnpoint coordinates
2. /worldborder to make a worldborder that the spawnpoint is NOT included inside
3. /kill and try to respawn