stroke width with expression

I played around and saw your issue. When you have basic shapes, you can create a workaround with a second - stroke only - layer.

If not, try the stroke effect.

*Martin

I have exactly the same problem, the stroke width of my shape stays the same but as I have a circle shape that becomes an ellipse when I squash it, the stroke is not even all around. As expected it's thicker at the sides of the ellipse and thinner top/bottom. I can't find a way to make it the same thickness everywhere.

If all that you're trying to do is keep a stroke the same size when scaling a shape layer, you don't need expressions. Just apply the stroke paint operation within the shape layer to a group that contains the scaling operation. This kind of thing is the reason that there is the ability to group shapes within shape layers and apply transformations and other operations at any level.