(EDIT. Found my problem was caused because the min y of -512 was added to the max height of 384 causing the world to only generate between y -512 and -128. User error I apologize for any inconvenience.)
When decreasing a custom world's min y to around -320 or below (for my case I used -512) the world just generates void or incorrectly. With the generation settings or the datapack it generates void but if you combine the generation settings with the datapack it generates just the deepslate.
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Just added a generation settings .json and a datapack for reference. but should do the same for any settings with a min y below -320 at -320 the top of the world generates flat.
Duplicate of MC-216699
Please include the data pack or generation settings used