When trying to export the superuser user being logged in as superuser, the export fails with
Write permission needed for export. User not allowed to WRITE path [/system/superuser]

Looking at the permission for superuser, the user actually has write permission for the users workspace with path "/*"

When creating a new user under /admin and assigning the superuser role, this user can export the superuser. So it seems that superuser cannot export its own user, since exporting the anonymous user works fine.

Edit: the new user cannot export its own user either.