I have the same problem. Get anywhere with a solution?

I too have the same issue... happens at the most random times :

I'm having the same issue.
I got it to work once by pressing the "import" button on custom models again. But that only worked once, and never again. It still snaps them into place when I do that, but I can't pack the map if i make edits or something like that because the zone is busy. Worse yet, if I keep messing with it, it eventually breaks and I have to reload a save, which I'd have to do anyway if I wanted to try to salvage the map.

One thing that kind of worked is if I have a save before it breaks, I can delete the objects causing the issue, reupload them, and repack. But this only seems to work for a limited amount of time.

In case it helps, its usually the same one object that always appears over the cloud-thing, however sometimes one or two more objects show up up there alongside the problematic one.

Lil update! I found that one of the problematic pieces wasn't even up in the sky! It seems to mostly be objects that have unique colliders and non-convex hulls. Right click some of your objects like that that are placed, go to "Custom," and click import again. Sometimes it can fix it!

For me this problem mainly happened with objects with non-convex colliders like 48787 mentioned, specifically complex buildings. It only **sometimes** was an issue though, and seemed to relate to the number of pieces contained in the map although map object count is not usually a problem.