Strange artifact on export

[Blighty]

I still don't see the problem.

I took your image "Screenshot of transformed & retouched image" and added as a new layer your image "Crop of exported PNG". I then used the Ofnuts tool Realign Layers to accurately align the two layers. The "Crop of exported PNG" layer looks better than the "Screenshot of transformed & retouched image" layer. But comparing a screenshot with a png crop is like comparing apples and oranges.

xcfgz file attached (with paths still visible). Use Gimp's File > Open to open the xcfgz file.