I was building a stylized tunnel in the nether when I discovered an odd glitch that prevented me from walking up the stair blocks was using. It seems like when the staircase is 1 wide and both sides of it are filled in with blocks (as in Block, Stair, Block) you can't walk up it from soul sand. It doesn't seem to affect wider gaps where the staircase is 2 or more blocks wide, just very thin tunnel staircases. After a few minutes of attempting to ascend I was able to go up when approaching at an angel but couldn't do so again after some more testing.