There were large glaciers/chunks of packed ice that formed underground in the caves below the glaciers that generate in the frozen ocean. They take up lots of space in the caves and based on the shape and position of them, it does not appear to be an intention generation.*

Steps to Reproduce:
1. Create a new world in the 21w13a snapshot
2. Locate a frozen ocean biome
3. Head underground to the caves below and the bug can be seen in the form of large chunks of ice in the caves