This ugly glitch seems to have been around for a long time
I can’t remember when I first noticed because I was otherwise happy to see they could walk under a staircase, so I didn’t really care about the weird shadow when they walked down the stairs. I was happy to see it seemed to have been fixed sometime between April and May this year I think we’re between updates…
Quote from patch notes, Update: 08/06/2015 – PC Version 220.127.116.110
Sims shadow detail once again includes extremities and not just an odd blob beneath their feet. Because Sims are more than just blobs to us… they are arms, legs, and the whole package.
But now it’s back 😦 although, I think it’s only an issue with interior staircases, because the second screenshot shows a staircase leading up to a flat roof which could mean it’s classed as an outdoor staircase.