When you generate a sea of icebergs in 1.17.1 or before, then generate new 1.18 chunks, the world generation seems to get confused, and generates new parts of an iceberg that didn't generate fully with the default grass, dirt and stone generation instead. In order to reproduce, go into a world in 1.17 or before, generate some ocean that has icebergs, and then go into 1.18 Pre 2 with that world. Any icebergs that generate past the point you generated will have grass and dirt generating instead of packed ice or snow blocks.