I've got some weird stuff going on with the tree substitutions. It's distorting them somehow I'm guessing based on the bounding box of the voxel instance (which is cubic).
EDIT: I'm so dumb. I circled the NAME of the Planting Rule, and not the asset name in the actual Voxel Instance.
Here is the Planting Rule, showing use of the Voxel Instance tree1. Note that I'm not using a Mesh Instance at all.
And here is the Voxel Instance tree1, showing where you need to make sure the Asset Name is the same as the asset you're using in Unity in your resources folder.