When copying SF Symbols, they are transparent.

I want to use some SF Symbols in a third-party app (It does not support reference by name, I am importing files).

What I'm doing at the moment is finding a symbol, right-clicking and selecting 'Copy Image As' with the following settings:

Copy As / PNG

Point Scale / 60

Pixel Scale / 10

Symbol Scale / Large

[COPY IMAGE]

Then I open Preview, and use File > New From Clipboard. Then I save the image.

But whenever I do this, the symbol image file has a small amount of transparency in it:

As shown above, the image opens in Preview fine, but has some transparency, which is messing with what I want to use it for.

Why is this happening, and how can I disable/avoid it?

Thanks,

Gill