Seemingly your PNGYour link https://i.sstatic.net/LeGdf.jpg gives a JPG with a background. That background is not your imagesingle color, but a screenshot of your workspace.See it here:
You should goto File > Export as > PNG to save
JPG cannot have transparency.Someone has converted your image to JPG. Be sure that all other layers are disabled butGIMP doesn't by default remove the one without acolor information from the transparent pixels when exporting as PNG. So when your PNG was converted to JPG, the original background can in theory be restored.