When a mountain generates in an old size world, the peak will be cut off, allowing the player to walk where there should be stone. If the player converts an old world to infinite the peak of the mountain will generate and kill the player if they are standing where it generates.
Linked issues
Comments 2
Merging this bug with MCPE-122043.
Duplicate of MCPE-122043.
I realize you're the one who filed both bug reports but they seem like the same bug tbh.