When placing stairs next to each other at particular angles to achieve the corner effect, some  stairs are able to have corners and others are not. The images will illustrate it better.

The first image (titled "1") shows that the stair is able to turn into an "outside" corner piece, without needing a stair on both sides. This was achieved by facing in the direction the player is facing in the screenshot, and placing the stair. However, the second screenshot (titled "2") shows that the player is not able to get a different stair in the same corner orientation, even when facing the same direction and placing it in an identical manner.

IMPORTANT: screenshot no. 2 might appear as if I just need to adjust the stair behind it to become an "inside" corner piece, and then the stair in question will fall into place (as in screenshot titled "3"). But even after orienting the stair behind in this way by placing the targeted stair in the way depicted in screenshot 3, the process of breaking this stair to place it correctly causes the stair behind it to revert to its state as in screenshot 2. And this is also a chain reaction which continues all the way up the chain of stairs.