Two graphic bugs with trees, both in this screenshot:
Trees can grow out of hillsides in weird ways and they don’t look right, just like they’re floating in midair.
If you build a slab on top of a tree, as the workers build the slab, they don’t show; the slab just mysteriously grows bit by bit (as you could see if this were a video – this slab is currently being “built”, but the workers are like mysterious ghosts and aren’t visible).
sooo, I think that negates this from being a bug… as it’s highly unlikely building foundations atop trees was an intentional design option… 
How else am I going to build a treehouse? I ask you!
You have “Slice” Mode on. Try turn it off, you should see your workers again. It’s the same if you build a normal building, they are hidden when you are in “Slice” Mode.
does he? it doesnt appear to be clicked on in the picture.
edit never mind its on but doesnt show on the icon for slice.
Ok, so it was still a graphics bug then 
I wasn’t completely crazy! It’s just that the icon wasn’t showing correctly.
Other bugs: if you build a house on top of a tree then remove the tree, the house doesn’t collapse.
