converting .usdz to .obj results in flattened model

I'm using the Apple RoomPlan sdk to generate a .usdz file, which works fine, and gives me a 3D scan of my room.

But when I try to use Model I/O's MDLAsset to convert that output into an .obj file, it comes out as a completely flat model shaped like a rectangle. Here is my Swift code:

let destinationURL = destinationFolderURL.appending(path: "Room.usdz")
do {
    try FileManager.default.createDirectory(at: destinationFolderURL, withIntermediateDirectories: true)
    try finalResults?.export(to: destinationURL, exportOptions: .model)
    let newUsdz = destinationURL;
    let asset = MDLAsset(url: newUsdz);
    let obj = destinationFolderURL.appending(path: "Room.obj")
    try asset.export(to: obj)
}

Not sure what's wrong here. According to MDLAsset documentation, .obj is a supported format and exporting from .usdz to the other formats like .stl and .ply works fine and retains the original 3D shape.

Some things I've tried:

• changing "exportOptions" to parametric, mesh, or model.
• simply changing the file extension of "destinationURL" (throws error)