Hi!!
I have made some more tests to try to help you with the diagnosis.
Please have in mind you have to be very patient to reproduce it. 90% of the time everything goes well, and then it happens. Sometimes it happens the first time you touchdown, sometimes it takes 30 minutes and dozens of touchdowns to see it happen.
I have spent more than one hour testing the helicopter in different terrains, and I have managed to see it happen in Gia Lam airbase (Cam Lao Nam), in the Altis airport, and in Tanoa. When in Tanoa, it happened on grass terrain.
So it doesn´t seem to depend on the type of terrain or the scenario. I would say it´s the helicopter itself.

Here´s a video showing some of the issues.

https://www.youtube.com/watch?v=bK-pQMdIQbE&feature=youtu.be

I will post a video about the slingloading problem, but it will take some more time because the frequency of the issue is lower.

When you add collective, you modify the angle of attack of the blades. The governor adds more fuel to the mixed gaz in the turbine, because the resistance of the air on the blades raises, and it tends to decelerate the rotor.
The action of raising the power of the rotor brings a reaction: More par effect, that has to be compensated with the antipar (pedals).

In fact, every movement of every control in the helicopter (Collective, cylic and pedals) needs some response from the rest of the controls.
Raising or lowering the collective needs an antipar compensation. It´s not a programming mistake. It´s a programming win for realism ;)