only 32-bit (i686) images shown for "Download an OS" on 64-bit (amd64) machine

originally reported as https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=895363

When I try to use the option "Download an OS" in the "New" wizard only i686 images are shown, though I'm running on amd64, e.g. see below when filtering for "debian", but all choices are for i686, too:

This happens with gnome-boxes 3.28.1, but also current head (75fa56ed), so the fix for #190 (closed) seems to not be working correctly.

Minor point: when I go back to the wizard's initial page and click on "Download an OS" again, it will list every image twice: