In Jira Cloud, I am able to update assignee/reporter through ROVO. But it always shows an error while trying to update custom fields with the user-picker type (same with single or multi picker). I get the below error,
Update Jira Issues for any fields (bulk) failed because ABTEAM-2: customfield_10076: One or more of the selected users do not exist or are not valid for this user picker.
I have tried this using the user's name as well their email addrss. But the same name works with the system user picker field.