This has always bugged me.

I created a box and round its corners.
Xara provided a Curvature that is the same for both H & V.
When you stretch the box the stretch from original is scaled into the H & V lengths so the box appears wonky.

@Xara - this is a Bug. the HTML filter renders the wonky corners as a border-radius (of the longest of H or V) so the corners are 'corrected'. Again, this is not part of your pixel-perfect mantra.

You can preserve the H:V ratio if you switch to the Rectangle Tool and change lengths there.
This might be too late in your design to recover the box back to its original size ratio.

All is not lost.

Click image for larger version. 

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Create a small square with rounded corners.
Copy the original box and on the small square, Paste Size and Position.
The square is aligned on top of the original with balanced H & V curved corners.
To tidy up, Paste Attributes and delete the original box.

The size of the square is immaterial, only the size of its corners.
Even if you use a square without rounded corners, you can select that option and resize the corners to the amount you need.

Acorn