TLDR;

• Don't use a empty state in your Animator Controller

• Use a static Idle animation as your basic "empty" building block state

The empty state will cause your character to fall.

The problem enlies in having the empty state in the Animator Controller or in my case, not generating a separate avatar for the idle_br animation.

I had a separate fbx (of the same model) with the idle_br pose and had the Avatar Definiton set as Copy from other Avatar, the value being my normal Master Chief generated avatar. I thought it was logical to copy avatar since it is the same model, rig, and bone structure. Instead you should Create From This Model and configure a new avatar.

If you take the robot model/rig for example from Unity's Mecanim tutorial and create the Mecanim avatar, automap, and enforce pose. Then make a new Avatar Controller with a empty state and set it as the animator controller. Then you will see the robot fall and go into a knees bent and arms out pose.