My house model in 3d (an .obj file) shows up fine in 2020.26 and before on the Layout Tab (and House Preview Window).
On 2020.27 and beyond (even to 2020.28), it doesn't show. The frame appears to be there (It's hard to see, but as I orbit around I can make out the edges of the frame), so it seems as if xLights isn't reading the .mtl file.
The only thing that has changed is the xLights version. If I use 2020.26 or before, it shows up as expected.
I've attached a screenshot of the issue in 2020.28, logs, the .obj, and .mtl files.
I'm on a Mac, so I'm assuming it's related to the sandbox update.