SQL Server 2012 Simple Recovery Model with LOG_BACKUP log_reuse_wait_desc
Alright, spent yesterday day and night investigating, testing and trying to reproduce the problem. Found the root cause:

MODEL database set to the SIMPLE recovery model.

If the model database has been set to the SIMPLE recovery model, and user databases are created with SIMPLE recovery model, SQL Server somehow is treating it as if it is in the FULL recovery model. Hence waiting for LOG_BACKUP to truncate the log.

I have listed the steps in this link to show how I reproduced the problem.

I think the root cause is if a user database was created (not alter after) with simple recovery model, then it has this problem. I blame it on model database as that is the only way to create a user database with the defined recovery model.

This documented in Microsoft Knowledge Base article 2830400 and is fixed in SQL Server 2012 SP1 CU4 and RTM CU7:


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