I built a bar that had a platform with a piano, mic, and guitar on it. I playtested it a few times, changed the lot type to a lounge so entertainer sims would show up, and they used all the objects as intended. Today I found out that sims cannot route to those objects anymore. After some adjustment, the mic and guitar was usable again. But the piano cannot be used even when it was the only object left on the platform. My solution for now is to just remove the piano. But I hope to see this issue fixed.
I figured out a work around. Place a new floor pattern on any space in the glitching platform. You may have to adjust your camera to be able to do so. Once you've placed the floor pattern, click and drag the floor pattern out one tile at a time until the entire platform is covered again. It'll be reestablished as its own 'room', and you'll be able to place the original floor pattern as well as objects on the platform again. Routing is also fixed.
I have the same exact issue on PC as moriisola, furniture snaps to the border of the platform and I can't place anything. The workaround I've found is fencing around the platform so that it's a room, then lowering it to match the rest of the floor. Then, I place everything where I want to put it and I can raise it back up again. I notice that it seems to happen more with higher platforms (for example, I was building a stage for a nightclub and the glitch occurred there). Hope this gets fixed soon, it's kind of important that we're able to place things on platforms--like it's the basic function.