Also, I am trying to reproduce the same behaviour on the original asset I started with, which is a lot taller, and I am running into an issue which I believe is connected to the scale of the character.
In the actual scene I’d like to test (the one I can’t share 
), Maya units are set to cm, and the character is about 180cm tall.
If I set the root’s “Translate Motion” to “Soft”, and the “translate stiffness” to “10000” (which I believe is probably exceedingly high). The root still moves away from its controller:
Even setting translate motion to “locked” does not appear to produce an effect. The root still falls to the ground (I’ve set the feet to “simulated”, for this second example).
rSolver scene scale is set to 1.02
These are the global preferences.
Translation and rotation stiffness limits are set to:
10000000000.000
Damping limits are also set to (now I can’t remember why/if I really needed to set the damping this way):
10000000000.000
(which might sound crazy high, but made the rotation limits work nicely).
I scaled up the limits because of what I read in this post.
For reference, the manikin scene has:
rSolver scene scale is set to 0.1
t/r stiffness limits set to 1000000.000
damping limits set to 10000.000
And the manikin is about 18 units tall (units are cm).
I suspect the translation stiffness limit and damping play a role in the root moving away from its controller, but I was unable to figure a solution out, yet.
At a first glance, do you see any red flag?