When a mountain generates in an old size world (use seed "42" for an example) they will be cut off by the height limit of 128, leading to phantom blocks that generate at Y=129 that suffocate goats and that can't be broken by the player.

Description from MCPE-122047:
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.