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The problem is mapshaper was rearranging your maps coordinates in the anti-clockwise direction and Amharts assume it arranged clockwise.
You can read the explanation and try python code to re-arrange it in:
https://www.wemcouncil.org/wp/how-to-make-mapshaper-output-compatible-with-amcharts4-using-python
it work for my case.
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For anyone else who runs up against this, I used this tool https://mygeodata.cloud/converter/shp-to-geojson and it was converted correctly from my ESRI shapefile the first time for AmCharts 5. Then I used mapshaper to simplify this output, worked great.
Edit: also found this tool because mygeodata is a paid solution. This one seems to be free and will not only reverse the coordinates of the geojson but will also check for any other errors (such as no "id" attribute).
Edit (again): Forgot the name of the site https://interactivegeomaps.com/geojson-tool/