It seems that if a chart has any values in gradientRatio-array, trying to save chart as an image will cause all slices to go transparent (excluding the borders) both on the page as well as in the saved image or pdf.
Everything goes fine if there are no gradientRatio parameter or if is an empty array.
You can verify this bug with your donut chart demo: https://www.amcharts.com/demos/donut-with-radial-gradient/
It seems that if a chart has any values in gradientRatio-array, trying to save chart as an image will cause all slices to go transparent (excluding the borders) both on the page as well as in the saved image or pdf.
Everything goes fine if there are no gradientRatio parameter or if is an empty array.