Physics instability with model imported from URDF with "Merge Fixed Joints" set to false

I have an urdf for a small vehicle robot and I have been experimenting with the URDF importer and have noticed that if I don’t have the Merge Fixed Joints box checked then when I start simulating the model disappears. I have attached a gif of me starting the simulation where one of the models in the world had the Merge Fixed Joints box checked and the other didn’t.

unstable

Also here is a tar containing the URDF and meshes,

test_robot.tar.gz (595.8 KB)