/setworldspawn in nether sometimes let player spawn in y=1

If set world spawn point in the nether by /setworldspawn, it sometimes let player spawn in y=1
reproduce:
1.going to the nether, find the ground and stand on it
2.enter /setworldspawn ~ ~ ~
3.enter /kill
expected result: respawn on the surface
actual result: respawn in the y=1 and x and z didn’t change (if that didn’t happen, try to walk for 2~3 blocks and re-enter /kill)