OBS:
Results vary depending on the instance of stairs.
In some object instances, AI approach the bottom half of the lowest flight of fireescape_01_tall_f.p3d (the part missing handrails) from the wrong angle, then clip through the stairs, then toward the correct direction of the stairs. They then realise they are still on the ground, so turn around and attempt to enter the bottom flight of the stairs again.

At other instances, AI just stop near the bottom of the stairs (pathfinding fail).

EXP:
AI should be able to enter and use all buildings as reliably as a human player.