(English is not my mother tongue, so please pardon me for possible spelling errors or unclear expressions which I do have tried to avoid when I am writing this issue report.)

Description of the Problem

I tested the command

/setworldspawn 94 -50 208

It did work, for the command did neither return any error message on client message box, nor print any error message in log on server-side. And then I stopped the Minecraft server, and checked the level.dat file on server, it's key SpawnX had value 94, key SpawnY had value -50, key SpawnZ had value 208. Then I restarted the Minecraft server.
So I think the /setworldspawn command is working properly.

However, I logged in the server with an account that never logged in before. This new player spawned at the void, kept going down and then disconnected from the server. I am sure this is not related to network issue, for the PC running Minecraft client was connected to the server through RDP at the meantime. Then I tried to log in again with that account, the player was at the original world spawnpoint whose Y coordinate>0.

Then I tested with a few other ID, none of which has changed its own spawnpoint with bed. I ran /kill command, and found that all of these players were teleported to the exactly the same original world spawnpoint.

Finally I chose a random point with its Y=20, and ran

/setworldspawn

at there. Then I tested where these players, which I used on the testing I mentioned above, would respawn. Testing method was the same. However, every player I tested can respawn at the new point whose Y=20.

With the same method, I tested this problem on Minecraft single-player name. It could be reproduced.

So I think setting world spawnpoint at Y<0 area is still not allowed in 21w13a, which is an issue should be fixed.

I am not sure what kind of attachments would help to resolve this issue. If any other information is needed, it will be provided as soon as possible.

Thanks in advance!