Legs movement when uploading a raise-sit anim

I did 3 similiar anim that start from a raised pose, sit down on a prop stool.
The stool is the same.
Started from the same frame 2, the one of the first anim. Only i changed the Hip starting up-down position.
I used Inverse Kinematic to let legs bend and feet stay (almost) still and it worked on anim 1 and 3.

In anim 2, even if my figure have feet on ground, when importing i see her raise feet from the ground when sitting down. Like she is doing abs during the sitting: the hip goes down and legs goes up!

In frame 2 i moved all body parts, hip included.
I tried to disable IK after finish and export BVH in this way.
I opened the bvh generated with QAvimator and BVHacker to see the skeleton and ground and feet are still on ground. But when uploading it jerks legs movement...

I though about the different legs lenght of my avi compared with the default figure that determinate a different movement, but in this anim the jerking is much much more visible than the other two.

Could be upload priority - you need Priority 4 to over-ride the default sit.

i setted priority to 4 as usual.

Another strange thing is that, different from the other 2 anims with the same starting position, in Play in World it runs 60 cm above the ground...much higher than the other two (at ground level more or less).

Dont know what to check again.

After trying everything, shins position, slower or faster movement, cheking every join, importing in other programs I found a solution: I dove feet into the ground in my animation scene and, for magic, uploading runs good.
Now of course in Poser, Qavimator and BVHacker the anims runs jerkly...

I was obliged to compensate the error made, i guess, by the optimizer.

However work this way it's like paint blind.