I wonder if that is a bug in Gimp 2.10 or intentional or was an undiscovered bug in Gimp 2.8
Try your indexed conversion using a maximum number of colours 2 (the minimum possible)
and a regular export to a png
Seems to work here, the colormap just shows black. I did open the exported file in a (linux) Gimp 2.8, still ok. Let us know your findings.
Edit: A bit more exporting / reopening in Gimp 2.10 - weird results Get back 2 colours in the colormap sometimes black/black sometimes a-color/black
These look ok in the Gimp 2.8 colormap and no great difference using ImageMagick identify -verbose. Must be a Gimp 2.10 bug
Try your indexed conversion using a maximum number of colours 2 (the minimum possible)
and a regular export to a png
Seems to work here, the colormap just shows black. I did open the exported file in a (linux) Gimp 2.8, still ok. Let us know your findings.
Edit: A bit more exporting / reopening in Gimp 2.10 - weird results Get back 2 colours in the colormap sometimes black/black sometimes a-color/black
These look ok in the Gimp 2.8 colormap and no great difference using ImageMagick identify -verbose. Must be a Gimp 2.10 bug