•  Symboles options: "Enable Guides for 9-Slices Scaling" —>   Limitations?

Hello -

For one of our customers, we have to work with a structure with a rounded corner.

See attached file please.

 
The problem is that some of my colleagues aren't too careful when it comes to deforming the structure (and often the corner is stretched so it's not a perfect arc).
To avoid this, we thought we'd make a symbol with the "Enable Guides for 9-Slices Scaling" option enabled.
This works well for deformation, but when we enlarge/upscale the structure, neither the thickness nor the curved/rounded corner is upscaleed.

 
Is there any way to prevent this?

So that the thickness and the rounded corner are enlarged/upscaled without this corner being strectched (ie. stretched lengthwise).

NB. Strucre is an exopanded stroke (so it isn't a stroke anymore)

Thank you

 - Dimitri