The world height has been decreased from 0 to -64, yet the command height limit has not been updated to reflect this. The spreadplayers command does not allow for numbers under values less than 0 still, stating that the number must be above 0 even though the world generates lower now.
To reproduce
/spreadplayers ~ ~ 0 1 under -1 false @sComments 15
Just saying, this is not specific to the caves and cliffs data pack, any data pack which adds or changes dimensions to have negative coordinates be valid, is affected. Might want to check values over 256 as well.
5 more comments
Can confirm in 1.17.1 with the Caves & Cliffs preview datapack. Likely the result the fix for MC-184699.