The work around for this is to move the staircase away from the landing. When you do, you will likely notice a section of fence appear where the cutout for the top of the stairs was. If you sledgehammer the section out where you want the top of the staircase to go then put your stairs back, it should solve the issue. Ever since platforms placing stairs against fencing makes the section at the top of the stairs invisible, but it doesn’t cut it out completely. So your sims routing sees it as still there and they won’t use the stairs. I hope they fix it and soon because it’s really annoying.
The other work around is to place stairs first and then add fences using the drawing tool, skipping the section right at the top step of the stairs.