Transaction log is full due to 'CHECKPOINT' for DB in simple recovery model
(since this is longer than a comment ... so putting it as answer)


  For one of our DB's on SQL server 2012 RTM.


This is your problem. There were many fixes introduced in the consecutive CUs / SPs after the RTM build.

Can you patch the sql server with the latest SP2 ? After patching, check if the issue still happens or not.

Your problem might be because your MODEL database might be in simple recovery.

If the model database is set to SIMPLE recovery model and user database is created with the simple recovery model from model database template, SQL Server does not truncate its log automatically like it suppose to (after a full backup). It appears that somehow SQL Server is treating it as if it is in full recovery model.

you can execute below log backup (even though your database is in simple recovery - as since model is in simple recovery - due to a bug in RTM, sql server treats it as being in FULL recovery)

BACKUP LOG dbName
TO DISK = 'dbName_log_backup.trn'
GO


Check this KB: Database does not follow simple recovery model behavior in SQL Server 2012 after you set the recovery model of the "model" database to "Simple"