I moved this post to our developer community where it's more likely to receive a response.
Can you check the Permissions on the setting tab , If site admin can add the users to the site.
I have also observed in 9 that site admins cant update the user roles even from front end UI, which they were able to do previously.
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You are correct - it appears that the permissions differ between the web UI and the REST API.
From the REST API Documentation:
This method can only be called by system administrators.