this is not the first time I report that here. But now I have news. I have a clue of how to do it on purpose.
After founding the reason of the bad result, it is easy to wrongly tell that: I had the feeling that the bad results come on scanned images. This was just a coincidence. In the last few years, I often crop logo (imagine a drawing of Superman, Batman, etc.) and put them in a OS folder background
I took a CD Image scan (I get it from internet), drop it into my Windows 10 Icon creator application on macOS (see another conversation) and the resized image was BAD.
I loaded it into Gimp, set the background to black, make a screen shot, save it to disk and try into the application: the resize was good: surprise !
I loaded the image into Gimp, select the surrounding black, invert the selection and copy it, paste it to Preview, Save it and then the result was bad !
Conclusion: if I drop an image that have been croped (remove surrounding white or other color(s)), the resize result will be bad.
If I bucklet a color on those transparent part, the resize result will be good.