REST API for Add actor users returns success (200) even if the user doesn't exist

Issue Summary

REST API for Add actor users returns success (200) even if the user doesn't exist. In the documentation for [JIRA 8.5.3|https://docs.atlassian.com/software/jira/docs/api/REST/8.5.3/#api/2/project/

/role-addActorUsers]

Steps to Reproduce

Do a POST request to the endpoint "/rest/api/2/project/<projectIdOrKey>/role/<id>" sending a body with an invalid use like this:

 {"user":["not.existing.user"]}

Expected Results

The documentation indicates "The project must exist and the user must have permission to view it.". If the user indicated in the body does not exist, the expectation is that there would be an error 404 as indicated in the documentation: "STATUS 404 Returned if the actor could not be added to the project role".

Actual Results

A message 200 (success) is returned.
The below exception is thrown in the xxxxxxx.log file:

Workaround

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