Biome placement by depth fails on Neoforge 1.21.1 down near the bottom of the world

[gniftygnome]

In Neoforge 1.21.1 roughly between depth 0.8 and 1, it's been reported biome placement based on depth fails and Biolith's describe command returns an error. I need to develop a reproduction and then suss out what is going on. Maybe also check Fabric to see if this is a vanilla thing or if I need to go read neo patches.