This seems to be a bug. We all know that swamps sometimes are very empty, with no items around.
So there's no dragonflies, no reets, etc.
However, I now noticed that boats and crates around a swamp can disappear.
Additionally, it seems that at least sometimes, they can reappear later. Of course the dragonflies will appear again later. But the same for the boats. So it is not a spawning issue.
So something about the code that loads the map is not loading the items that are actually there.