Export as Indexed PNG's alpha channel can be more than 1-bit
Problem: after I load source PNG file (social-widget.png, attached) into GIMP, it garbles its alpha channel, making the smooth rings edgy. The second file (social-widget-processed.png) illustrates that.
How to re-produce: open the source PNG in GIMP 2.8.22, and export it into another PNG file.