The Bug

The specific superflat presets which are intentionally designed to resemble the Default world generation have not been changed despite the default world generation being significantly changed. Namely, you spawn 64 blocks too low now because an additional 64 blocks of stone do not spawn to compensate for the new world depth. (Additionally, Tunneler's Dream could have even more than 64 layers of stone added being that it is designed to take up most of the build limit for... tunneling. This particularity may not be a bug but it would make sense.)

Some, such as Classic Flat, should not be changed, as well as Bottomless Pit and The Void. Redstone Ready and Water World do not matter either way.

This can be fixed by the player by changing the presets, but this is not ideal since there is no save function for custom presets, and it essentially breaks the purpose of having presets.

How to fix

Update the old settings to the following settings to comply with the 1.18 update:

minecraft:bedrock,64*minecraft:deepslate,230*minecraft:stone,5*minecraft:dirt,minecraft:grass_block;minecraft:windswept_hills

minecraft:bedrock,64*minecraft:deepslate,59*minecraft:stone,3*minecraft:dirt,minecraft:grass_block;minecraft:plains

minecraft:bedrock,64*minecraft:deepslate,59*minecraft:stone,3*minecraft:dirt,minecraft:grass_block,minecraft:snow;minecraft:snowy_plains

minecraft:bedrock,64*minecraft:deepslate,3*minecraft:stone,52*minecraft:sandstone,8*minecraft:sand;minecraft:desert