Skeletal mesh Physics Constraint Projection settings omitted when collision takes place???

Hello, can’t understand a reason for such physics behaviour, solving it for 2 weeks…
Need your advise, where to dig.
I have a rod of 40 bones in a chain, each next one is less then parent one, like a fishing rod. A cube is connected with a constraint to the top of the rod.
While the cube hangs - everyting works perfect.
But as soon as the cube touches someting - someting happens, like the Projection settings for the top of the rod are totally omitted by physics of th engine:

The cube is hanging:
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It touches the surface - the top of the rod breaks, like Projection is set to 0 for the last bones!
изображение

and it keeps broken while the cube touches something, even if I drag it along the ground to create some force. If I lift the cube - all is good at once again.

Thanks!

Anybody please…

Already half a year I’ve been trying to solve it.
All bodies do simulate physics, none sleeps. No break events occure. It behaves like something just stops simulating physics as soon as it touches some other physics body.

Please, has anybody just any adviсe?