Are you changing more than one stroke at a time. And are they different weights to begin with? Is Use Preview bound turned on?
if so try deselecting use preview bounds in the preferences..
In the video I am changing the stroke strength of all four shapes at once. But this doesn't matter in this case because all shapes had the same stroke.
It's so weird... I tried your tip with the "Preview Bound" but that didn't help. But it seems as Illustrator is internally using the preview bounds with a "virtual anchor point" in the top right corner (when increasing stroke width) and another anchor point in the bottom left corner (when decreasing stroke width). The shape itself tries to center its position.
I just recognized that the shape moves -0.5 on the x- and -0.5 on the y-axes each time I increase or decrease the value of the stroke strength by an odd number AND only when the stroke is aligned on the center of the edges.
If it's an even number the coordinates remain as they should.
But why does this not affect ellipses for example? Same configuration, same settings, same artboard, same stroke, same fill... But NO moving towards the bottom left corner...
UPDATE: I tried the same with the star shape tool from the panel. The star is not moving to left but to the bottom. And what's even more strange is that the original HEIGTH of the shape changes. I mean that Illustrator is distorting my original shape! ;(
Now I tried to draw a simple rectangular shape and changed its stroke strength ==> it moves to the bottom left corner as I previously mentioned.
But if I rotate the rectangle and then change the strength of the stroke ==> it behaves as it should an doesn't move. Rotating it back to 0 degrees and changing the stroke again results in the reported negative x- and y-movement.
This is caused by the "Align to pixel grid option". If you don't need it, you should turn it off.
Thanks Monica I knew there was a setting.
some pixel magic
Thank you so much!!!