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magick -size 600x10 xc:black -background none -wave 5x30 output.png
The same command using "convert" with IM 6.9.8-10 creates a wavy black line on a transparent background, which is what I would expect.
If I eliminate the "-background none" with either IM6 or IM7, it produces a wavy black line on a white background as expected.
With IM 7.0.6-1 and using "-wave" with any background color that has any alpha transparency, even like "-background #FFFFFFFE", outputs a completely transparent image. This looks like something that might require the attention of the developers.