At the moment, some stairs blocks cannot be used by the player when another block is placed over the downstairs side. If the player is walking down the stairs, they almost reach the bottom, then the player's head collides with the overhanging block, and they cannot pass under. Based on the height of the player, they should be able to do so.
This bug has been present on every version of the game I have played, including before the recent visual overhaul.
It means that a 2-block gap must left above the stairs in order to use them. That often makes it really hard to build sensible stairways in towers or small spaces. Another user posted about this problem here.
I have uploaded a blueprint to the Steam Workshop that demonstrates the problem. Please use it to test it out and develop a solution for us.