not seen that problem myself but you could save the image as a tif file and then select alpha channel when in your image editor, copy paste that into another window as your png, you have the tif as a backup that is easy to extract the png from

Tools:- Win10, Dell XPS8900, ZBrush, Marvelous Designer 11, Hex 2, PSP8. PSP 2019 Ultimate, DAZ Studio, Affinity Photo, Affinity Designer, Filterforge 11, flowscape,  Classic UVMapper, and several headache tablets.