Object Selection tool>Cut via Layer - causes a halo to appear around the cut object, you can reduce the effect using Contrast, but how to completely get rid of this halo?
What good is the resulting white area supposed to be?
If you »Layer via Cut« a pixel at 50% transparency the result are two Layers with a pixel at 50% transparency – that is not equivalent to the original pixel.
What you are observing seems to be as expected and intended.
You could
• abandon Anti-aliasing and Feathering for the Selection