MC-214951 was regarding the dark horizon and has been fixed, please do not mark this as a duplicate. Also, please do not mark as a feature request, because . Thank you!
Since deepslate has not been added to superflat at all, it spawns you in negative coordinates. This, for some superflat presets such as Classic Flat, is not a problem. However, it breaks many presets such as Overworld, Snowy Kingdom, and Desert. This causes the ores to fail to generate being that the superflat presets have not been updated to generate deepslate and ores do not generate in stone below y=0.