No, no external forces. All internal forces.
Ok, as assumed.
This is a fundametal principle of nature.
Yes, but i clearly saw all laws of nature being violated by the simulation.
Again this part is working.
Maybe - due to to the above - it is not working. Or something else, eventually causing you confusion and looking at the wrong things.
I know Newton usually behaves correctly. But not always. I often saw it breaking the conversation of momentum but adding energy when working on powered joint ideas.
One example was the hopper joint which is both ball socket and slider, which worked only pinned to COMs of bodies.
Another was a joint which can both rotate and position a child body at will, which did not work at all.
The issues i got looked similar to what i see on your current model.
To test this, when i worked on my own physics engine, i always did this:
Disable gravity and make the motors move. If the averaged com of the whole model stays in place, i'm fine. If the model starts moving into a certain direction, something is wrong.
Same for angular motion: If it starts to spin and keeps spinning once the motors are off, erroneous external force must have caused the net spin? (i'm not 100% sure about this angular claim to be right)
I keep pushing on the topic because you seemingly assume i talk about issues emerging from the control method, which i misunderstand. But no - the issues i saw are real and they certainly violated laws of physics way beyond any acceptable tolerance of error. It was so obvious i did not even mention them initially, assuming you see them too, are aware and work on it. But from your responses i conclude you're not aware.
Oh - sorry! I just run the demo again, and this time it ends at a valid state!
It takes much longer till it stops than i remembered (about 30 seconds), but all looks fine.
So now i see why our discussion diverges.
But the issues were there - i assure you. Will try some stuff as i remember having done, maybe i can reproduce it...