Found in Version:
Caspering in OE: in_c_pillar_wood, in_c_djamb_rp_arched, in_c_djamb_plain_square, t_imp_setmw_i_pillar_01 (nearly every house in OE seems to suffer from this). Considering how widespread the issue is, maybe the meshes should be edited to be solid underneath. Simply enter a random OE house and look up the stairs, most of them will have this issue. Alternative solution is to fix each individual house.