We recently discovered a bug related to placeables and decay that may be the source of this issue.
If you put down a placeable and then place building pieces around it, the building piece keeps its old decay score, separate from the structure around it. In other words, if a placeable has 1 hour left on the decay timer when you create it, it will still just have 1 hour left on the decay timer even after you’ve built something around it.
If the structure is made first, the placeable will have the correct time on the decay timer.
We’ve put one of our coders to fix this 