Scaffolding blocks on top of corner stairs all underwater cannot decide to stay a falling entity or a block, constantly glitching through and wobbling into the stair and reversing back to a block, but targetable when aimed at.

Appending upwards work as normal with the scaffolding, and you can swim up through the scaffoldings underwater, but you cannot elevate consistently above water inside the scaffoldings, as it exists as a block some of the time and entity some other time.

Breaking the bottom scaffolding does not destroy the entire scaffolding structure, but allows the scaffoldings above to fall and land on the stair the same way as sand does, and back to wobbling.

Untested with other stairs.