Resample after a color mask cut and paste retains cut color in antialias.

In Photopaint :

The image is mostly black and white. I use magenta to fill around the outside, then select magenta as the mask color.

Then I copy the reverse of the masked area and paste to a new image.

So I have just what I want in the new image, (the previous image less the magenta fill) - no magenta to be seen.

but when I resample, with alias on, a magenta outline appears.

I assume this is because the magenta area was not actually cut out, just made invisible by the app.

Is there a way to actually remove that masked out area?

or some other way to prevent that residual magenta interfering with the resample?