Stage 1 - Set first vertex point before editing the path.
And what else? What specifically is your issue?
I am asking for information. If no 1st vertex is set, why do the poiints behave in the insane way they do, compared to editing any other mask, or shape in other software?
AICB paths simply can have an arbitrary point order. That's an age-old limitation of how this stuff once was incepted. You can find traces of that even today e.g. in the Illustrator SDK docs. I know it sucks, but Adobe simply has never cared to develop something better. It would benefit multiple programs, after all. In AE the situation is only particularly bad because those paths are not consistent with animation. There's really no ill intent or other mystical stuff behind it, it's just 25 year old code.
I just wondered if there was a way to understand it, as I wasn't pasting in any new paths, just moving the existing points of a native AE shape layer, converted from rectangle to bezier path first.
It held it's original shape, scaled down centrally then switched to the final shape which was only slightly rotod.
I think just setting first vertex as a standard first step usualy makes things work.
Sometimes I want to peek behind the curtain, but usually it just scares me!
Knowing the programming behind can help use, but as I don't code, it can be hard to understand it anyway.
Especially as things are changing!
Thanks for help