Bell does not support AB412... I guess you get what you pay for
Back to the problem; what you are seeing is the infamous "tennis racket" effect, the tendancy of the blades to feather, which in some OAT and DA conditions, can be greater than what a single hydraulic system can take. Easy way to proove this is to roll the throttle down to 95% Nr and see if you can bottom the collective.
Another issue can be out of wack autorotation RPM adjustment. On a 412, you should see around 22 to 24% Tq, flat pitch ground, 100% Nr.
Let us know how it turns out & good luck
