The spawn changes position in old world

Steps to reproduce:

1. Open World 1.16.221.mcworld
2. Die to see spawn point

   /Kill @e [type = player]
   

Expected results:
The spawn point remains the same. In the given world, the generation point is 128 ? 128

Observed results:
the spawn point changed.

Note:
If the world has become infinite before entering, the same generation point will continue to be preserved.