An Error or Warning should be shown in the SDK build window when a spawn transform is found to be at or below the respawn height y value. This can be done on accident and isn't something that would be obvious to end users (even with the new respawn plane handle).

Especially since it causes the Quick Menu to close, meaning you practically soft lock the client if all respawn transforms are underneath the respawn height. I imagine quite a few creators would not be able to make the connection that their player being locked with no access to the Quick Menu means they are being respawned constantly.