Sims won't go up stairs

Yes, that has been an issue since the platform update which they did in november. So it's been a while now. As you said, removing the fence will often fix it. From what I understand the issue is that sims no longer can use stairs that go to a room that have floors removed, so removing the railing often fixes it because that removes how the game treats it as a room in build mode and it becomes an area with walls.

Here is the bug reported on Answer HQ: https://answers.ea.com/t5/Bug-Reports/WORKAROUND-Open-staircase-cause-routing-failures/td-p/9716427