Thanks Benjamin,
I’ve taken a look and there is a definite setup difference between the normal manny type physics asset and the one you are using. It looks like most of the constraints have been left as default settings which won’t resemble how joints actually move, so the character just crumples on a heap. This will affect convergence a lot since there is now a large interconnected pile of bodies folded around each other - that is terrible for convergence (which is needed for an island to sleep).
I’ll raise this as an issue on a public bug tracker, and post back here. In the meantime you can improve the stability by copying across some more sensible joint settings from the default manny physics asset.
All the best
Geoff