Paint Shop Pro :: White Glow Where Image Should Be Fading When Saved To PNG?

how to fade to transparency by myself I am left with a residue problem when saving to a different file format.

I took my square image and pasted as a layer. Behind this is a raster layer flood filled with black (As the image will be going onto a black t-shirt). I used a rounded rectangle selection tool to make a mask somewhere just inside the image borders. I then applied a Gaussian blur to the mask and it faded the edges of the image beautifully. I checked with the black background layer off and again it faded nicely into transparency.

Now my issue is that when I save it as a PNG file (with background set to transparency of course) and try to use it against various coloured backgrounds there seems to be a white glow around the edges where it is meant to fade out.