Hey there.
As of quite recently (the last few days), an exact email match is required to find and add users.
Previously, the SonarCloud API allowed any SonarCloud user to read all SonarCloud user data. This created some GDPR concerns (first and last names were being returned as well), and we mitigated those concerns by requiring an exact e-mail match and hiding the name of the e-mail address owner.
Your feedback is welcome (good or bad).