Hey, thanks for trying out the plug in!
When you open an exported file (a slice). Do the black bars go away? I had this happen to me on Windows, the icons look like they have black bars where there is transparency but the actual export does not. Can you check this for me?
If there is still an issue, can you also attach some of the exported slices or even the aseprite file? (maybe you can DM me the file if you want)