After correctly using Gimp's Color to Alpha feature to turn white (or other colour) background into transparency. It appears also correctly in Gimp. However, every time export the image, there is no transparent background, instead the white background appears.
Export image to PNG format instead of JPG
In case the image to be edited is in JPG format, then whenever it is to be exported, GIMP makes the default format same as the original, i.e. JPG, and you can never get the expected transparent background even the process of editing is correct.