Started a new world using seed 10, and just running along and exploring I found an igloo had generated on the top of the frozen river.
The ladder down to the villager/zombie room is flooded, but the room itself is not flooded.
It doesn't seem like an igloo (or any structure) should form on the frozen river.
Thank you for your report!
We're tracking this issue in MC-214795, so this ticket is being resolved and linked as a duplicate.
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