The structure minecraft:woodland_mansion/1x2_d_stairs contains two blocks that make the structure asymmetrical. In the screenshot attached, you can see that on one side, there is a birch stair, and on the other, there are two birch planks. Both the stair and the blocks are outlined in the screenshot.

Besides these two blocks, everything else in the structure is symmetrical.

(By the way, I removed some blocks so that I could take one screenshot for all of this. I forgot to remove the cobblestone block on the right, so it looks asymmetrical. Sorry!)