Acabei de descobrir que esse comportamento é intencional:
Cause
This issue is by design. The email address that's stamped in the root folder is determined at the time of profile creation and should reflect the primary SMTP address that's stamped on the user. The email address is strictly cosmetic and cannot be changed when the user's primary SMTP email address changes.
Workaround
To work around this issue, delete and then re-create the profile for the user.