Strange artifact on export

[wildisthewind]

Well, I found something useful: I'm able to get GIMP to match the exported image by unchecking "proof colors" in View>Color Management. So whatever "proof colors" is doing was obscuring things when I made the transformations. I'll go look up that menu item and see what I can learn about it, but if there's any way to have that "proofing" applied during the export, maybe I can get away with having worked under those display settings.