Post any defects you find in the released or beta versions of the ImageMagick software here. Include the ImageMagick version, OS, and any command-line required to reproduce the problem. Got a patch for a bug? Post it here.
magick Capture.png -alpha off -fill "#FFFFFF" +opaque "#000000" Capture7.png
Perhaps this is an issue of starting with a 3 channel image and resulting in an image that has only black and white and thus would be a single channel result.