I've noticed that the Perspective Transform uses opacity on the transformed result to deal with edge distortion.

Is this done with an external library function or was it developed as part of krita?

Given that it happens and I can see it happening, I can do things to work around the effects but I was wondering if it's unavoidable. I've used perspective transform in krita before with no apparent problems and this is the first time it's happened. This seems to be because I've used a more 'extreme' amount of perspective distortion on the example images shown in this post.

This screenshot https://imgur.com/bLDvnNl shows a preview of a transform (on the right) and you can see it 'has no problem'. In the middle is a previously done finalised transform of the texture on the left.

This one https://imgur.com/F2MVXs3 shows the end result after the transform is finalised. The red marks are to reveal and emphasise the transparency.

In this one https://imgur.com/YIukxQ3 I've split out the transparency and you can see the serious geometric distortion on the finalised result.

A quick test with GIMP using exactly the same image sizes (via png export) https://imgur.com/xW9SMv0 shows that GIMP doesn't have this problem. I have no difficulty mixing GIMP/krita usage for different things, if I feel I need to, but I'm surprised that krita doesn't do this well.
Given that krita can show a very nice looking preview, it's strange that the final result is the way it is.