This is a duplicate of the now resolved MC-243079 bug report which was hastily closed as "Works As Intended." I believe that bug was not well reported and deserved further consideration.

It has been a well-known mechanic since 1.13 that /setworldspawn will allow players to respawn as close to the set y value as possible, instead of on top of the highest block (like in 1.12). This works regardless of the spawnRadius set. From my brief testing, it seems to require at least a 1x3 location at any y value. If that is not met, the player is respawned at the next highest block. 

This behavior has been reverted to 1.12 behavior sometime in the development of 1.18 (which is unfortunate considering the stunning new cave generation making underground spawn points more enticing). /setworldspawn will now always pick the highest block in the column (including leaves, unlike 1.12).

Steps to Reproduce:

1. Open a world between 1.13 and 1.17.1 and another in 1.18.1 (and another in 1.12 if you wish to compare)
2. /setworldspawn
3. /fill ~-20 ~10 ~-20 ~20 ~10 ~20 minecraft:stone
4. /kill and respawn
5. Repeat at any y value of your choosing with at least 1x3 space to spawn. Invalidate spawn location and the 1.13 - 1.17.1 game will pick the next highest valid location to respawn the player.