I'm posting this in hope of saving anyone who is learning to create maps a bit of a hassle.
If you place a stair content tile be aware that you must also place a floor tile under that stair or it will not be navigatable by units.
(https://openxcom.org/forum/index.php?action=dlattach;topic=11275.0;attach=59845)
(https://openxcom.org/forum/index.php?action=dlattach;topic=11275.0;attach=59844)