xdg-dbus-proxy is now sandboxed, but it will only be mounted in the sandbox if it's installed into /usr/bin. If the prefix is anything different, then it won't exist and will fail to launch. Oops.
Created attachment 432044 [details] Patch
Committed r279215 (239101@main): <https://commits.webkit.org/239101@main> All reviewed patches have been landed. Closing bug and clearing flags on attachment 432044 [details].