When an old world is upgraded, and that world has icebergs at the edge of the blended area in a frozen ocean biome, the icebergs get dirt and stone placed next to them.

To reproduce:
1.Make a creative world in 1.17.1 with seed 16. Save without moving.
2.Load this world in 1.18-pre1 (or version being tested).
3.Fly around the edges of the world around the frozen oceans. -> Icebergs have stone and dirt placed on them at the blended edge.

Attached screenshot was taken with seed 16 near -97 -270.