Pocket spawn point high in air or underground in cave

My initial spawn point is on top of a small hill. However initially whenever I died I spawned way up in the air above the spawn point, come crashing down & die. When respawning after that, it was at the correct location.

Now, however, whenever I respawn, its in a dark cave below the spawn point. Its full of zombies & skeletons & I die instantly.

I've done a lot of work on this world which I now cannot use due to the unusable spawn point.

In other words, the respawning code is not respecting the vertical coordinate of the respawn point.