It is a problem with your install, in my opinion, because your IM does not find webp when you run
convert -version
or
convert -list format
I know nothing about installing on Linux. But my method of installing on my Mac clearly works and shows webp listed from both commands. Granted I install IM from source and not binary. So there could be a binary install issue from your Homebrew, if that is how you installed IM. Did you install IM from Homebrew +webp. Or did you rely upon some already installed webp in a place that IM does not find. If I recall you installed IM from Homebrew and I doubt that in comes with webp. Homebrew install to /opt. But perhaps your install of webp went to the default /usr?