From this thread (hope this sort of post in this forum isn't getting annoying): /139/fd/127951/1.html
I've seen this behaviour recently, I think it's related to sim load. When creating or editing objects in some sims, there's a delay in loading the object data, and in some cases it doesn't load at all. Unfortunately, it appears to happen at random, so I can't really give a good reproduction of the bug, but it doesn't appear to be a system change.