In the "deep frozen ocean" biome, packed ice will generate below the ocean, deep into the caves. However, this generation stops suddenly at y=0. Not sure if the ice itself is a bug, but the cutoff definitely seems like it is.
The world shown here has seed 323093351240230573
And the cave is at -1112 -6 -1660
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We're tracking this issue in MC-214823, so this ticket is being resolved and linked as a duplicate.
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