impresario_cci login vanishing

Last week we had a couple of users who were receiving the following error when trying to open the extractions module.

Extractions Database Connection Error: -1SQLSTATE = 42000
Microsoft OLE DB Provider for SQL Server
Cannot open database "impresario_cci" requested by the login.  The login failed.
[m_main_normal.m_extractions.clicked.45]

When I looked at Security>Users in SSMS for the impresario_cci database I could see that the users did not have logins for this database.  In the Tessitura security module I dropped and then created logins for the users and this resolved the issue.  I could now see logins for the users under the impresario_cci database in SSMS and they were able to open the extractions module without issue.

Today, one of the users reported the same issue.  When I looked at Security>Users in SSMS for the impresario_cci database I could see that again they did not appear in the list of users for the impresario_cci database.

Dropping and creating a new login for the user in Tessitura Security resolved the issue again, but I am concerned that this may continue to happen.  Has anyone else out there experienced this issue?  Is there something that we might have missed in setting up the user that is causing it?