I had a similar problem recently after updating to Xcode 13.2.1 and a MacOS update. My Safari webextensions in development weren't showing up any more in Safari (even with unsigned extensions enabled).
I think it turned out to be a problem with LaunchServices and Safari because running the following in a commandline/terminal fixed the problem:
PATH=/System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support:"$PATH"lsregister -f /Applications/Safari.app
Then my extensions started showing up again instantly.
nixoloco
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