This image should explain it. We entered a slash in the platform name, and it was accepted. This created a new subdirectory, which is not what we want, and will cause problems later.
Closed: won't fix
3.0
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Hmm...but it's totally valid to do this, isn't it?
I'm going to close this, but we could potentially re open and add it as an option somewhere, to prevent platforms going into sub directories, or at least prevent hosting platform pathauto from generating paths with a / in it.