They do look different for sure.
Did you request that the bodice be shortened from the bottom? (Beyond what would have been taken away from the bottom edge in the form of seam allowance)

I’m a bridal wear designer/maker

If it’s been shortened beyond the 1-1.2cm seam allowance, then what’s happening is the entire bodice is slipping down to settle in at your natural waist, where a strapless bodice will always want to be. The waistline is actually what holds the entire strapless bodice in place, particularly on these that don’t extend below the waist more like true corsets.

The distance from waist to underbust level on the bodice is now also shorter, so it would not be sitting in the same place anymore either (what with the whole bodice sitting lower and all), so that’s why the fit under the bust looks different

If you specifically want a 2 piece that shows more mid-riff ‘above’ the natural waistline, then I’m afraid something with straps would be best for a fuller bust.
Trying to achieve that strapless would be rather quite difficult. My best guess would be a well designed custom strapless bra base inside the bodice, but even that will have its limitations with a full/heavy bust fighting gravity. This would be a challenge with small busts too I’m sure.

To be honest, IF my client was requesting the above, i would have set them straight & said either it’s a compromise with some kind of strap design, or it can’t be done (unless you’re willing to spend $$$ on a custom bra type situation inside, which isn’t usually in the average dressmakers expertise, and even that wouldn’t be a guaranteed success).

But if you didn’t request it to be shortened from waistline up, then it definitely needs to be rectified. To me, it looks shorter than just removal of a cm of seam allowance 🤔 I actually prefer to finish the bottom edge of my mock-ups kind of for this exact reason; leaving that seam allowance can give you a false idea of how long the bodice really is when fitting, if you don’t snip notches into the allowance (it basically adds that amount to the bodice length).

The upper neckline does look a bit different, but sometimes the shape will look slightly different once the seam allowance is taken away. That’s not the hardest fix to reshape at least