Sometime when you recover wreckage there is a bush or young tree under it. When it’s a young tree it appears to destroy whatever it was that was in the wreckage. I’ve tested this by digging up an isolated wreckage tile and afterwards there was nothing but a young tree there.
Smashing the young tree resulted in only sticks.
The items from wreckages are added when the wreckage tile is added, at this time the tree is already there and prevents the items. The items are moved (at that point during mapgen) to a neighboring tile, one of those should have more items than usual.
And if all the surrounding tiles are young trees?
Wreck items spawns could probably be moved into deconstruction, smashing or something like that. It makes sense that a wreckage could be slowly removed without a shovel (20 minutes for something shovel does in 2), that explosions could scatter the wreckage etc.
And if all the surrounding tiles are young trees?
The games tries to find an acceptable place in a 2 tile radius. It will only fail and forget the items if there is a 5x5 area of tiles that don’t allow items and the new item was going onto the center, a very unlikely situation.