This command produces the following result...
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magick wizard: -background none -swirl 90 result.png
Rather than just properly swirl the image, it creates a moire pattern of mixed magenta, yellow, and transparent.
The problem only occurs when setting "background none" before "-swirl", but not when setting the background to any other color. It seems to affect any output format, not necessarily with transparent areas of course, and it happens with any amount of swirl.
The problem still occurs when setting "-channel RGB" or "-alpha off" ahead of the "-swirl" operation. Interestingly enough, it doesn't occur if I use "-alpha set" ahead of "-swirl".
The immediately previous version, 7.0.8.-59 had the same issue. My most recent version of IM 6, ImageMagick 6.9.10-11 Q16 x64 HDRI, doesn't have this issue and produces expected results.