I think we are talking about several different aspects of the very same system. The movement of your first video is quite usual, i can replicate it every time without turning the atom off and on. I am sure that the reasons for this is everything we discussed before, it is the whole system, especially important are the node properties. I can replicate wild movements every time if i want (for example with comply setting on specific nodes).
Yes, I also believe that smaller movements are normal.
And as long as they are minor and do not destroy a functioning animation, this is not a problem.
But, when I click Use Current Pose in Timeline or later Overwrite Pose, then it fixes the position completely.
When I then switch between the animations, there is absolutely no movement anymore.
In my second Video, there is a little movement, but that is just, because I haven't clicked on the Overwrite Pose button again.
After I do that, nothing moves anymore, when I switch the animation.
This will remain so, until I toggle off and on the Person Atom.
Also, in your first video you can easily fix every single "wrong" movement using the "keyframe pose" command from the advanced menu or changing the node properties. I tried this a dozen times with a fresh scene, and it is always the same, it is reproducable. Then even turning on/off the atom doesn't change anything anymore.
I never used the Keyframe Pose button in the advanced menu.
What exatly does it do?
I found something interesting: i screenshotted every pose and change and examined it later - in your first video the first pose after changing the animation is a wrong one, the second pose is the correct one. So this mini movement is needed, it is actually correcting the current pose to the keyframed ones. Didn't expect that. But this is just a fun fact and not applicable to every situation.
Unfortunately I could not make the same observation as you.
In my first Video at position 0.06, where I toggle the Person Atom Off and On, you can clearly see,
that the first pose is the right one and the second pose after the mini movement is the wrong.
I also checked this with screenshots out of that video and came to the same conclusion.
And when I press the "Apply saved pose" button in timeline, the saved position matches the first position before the hip rotates away.
It's strange somehow and would make more sense otherwise
Your second video is different, but i guess the reason still is the complex system behind it, perhaps addtionally complemented by the circumstance that newer timeline versions may have a problem with keeping persistence regarding node properties when switching animations - which would explain your scene perfectly:
"In Timeline 5.x, the Position/Rotation state toggles (On / Comply / Off) on controllers don’t persist when switching animations." (timeline discussion, last page). This could be one missing puzzle piece
I also think this might have something to do with the newer versions of timeline.
I really can't remember having this problems in earlier times.
Something seems to change, when toggeling the Person Atom Off and On and messes something up in Timeline.
That you can fix the situation of your second video by setting the node property, take us back to the beginning of this discussion, although it is always specific what you need to set. In your first video setting nodes to "on" regarding position and rotation also helps fixing these little movements in lots of cases. As did "comply" for me in my previous scene.
It is strange, that changing the hip node propertey and then toggeling off and on the person atom fixes the problem.
But I still don't think that the main problem here is the tensions between the nodes, I guess, this would not lead to such big movements
or at least would go from the wrong pose to the right one and not otherwise.
For me it looks like (Use current pose) or (overwrite pose) in timeline fixes the pose completely.
Then toggeling off and on the person atom makes this fixation undone and messes something up in timeline.
Then change hip node state and toggeling off and on the person atom again reverses that undone fixation and every animation is in it's correct pose again.
The only thing not clear for me: though turning on/off an atom isn't the main culprit of the general problem, in my dozen tests it sometimes (there were only very few cases) created that movement as in your second video. I guess the process change/refresh something of the properties we discussed or something like a neutral status is loaded and then things applied lead to this.
It is interesting that you have this extreme case, as it is on the second video, only in a few cases.
In most cases, I have these big movements that cause the animation to stop working properly.
If the movements would be minimal and not affect the animation, it wouldn't be that dramatic
