When a pre existing user with basic role is invited with site administrator role intermittently throws 'Unable to add' user error

Issue Summary

When a pre existing user with basic role is invited with site adminstrator role, it intermittently throws 'Unable to add' user error, and the user is not converted to site-admin; even though the user is added to each products' group.

Steps to Reproduce

1. Invite a user with basic role, and provide a product access.
   
2. Invite the same user but with Site administrator role
   

Expected Results

Despite the user already existing in the site, the user should be updated the most recent invited role and product access.

OR

If the user already exists, then do not allow the user to be invited and show message explaining the user already exists.

Actual Results

The page throws an error 'Unable to add <email>'.

The user's role is not updated however the product access is updated.

The below exception is thrown in the xxxxxxx.log file:
Status Code: 412
{"key":"preconditionFailed","context":{"message":"PRECONDITION FAILED"}}

Workaround

Currently there is no known workaround for this behavior. A workaround will be added here when available